The Virtual Climate Modelling Summer School covers the fundamental principles of climate modelling. The school is run for 2 weeks in the September of each year by the leading researchers from the National Centre for Atmospheric Science (NCAS) and from the Department of Meteorology at the University of Reading. I attended the school mainly because I have recently started using climate models in the second aspect of my research work and also because one of my supervisors recommended this to me.
What happened during the first week?
In the first week, lecturers introduced us to numerical methods used in climate models and we had a practical assignment implementing a chosen numerical method of our own in Python. We mostly worked individually on our projects that week. There were also lectures on convection parameterisation and statistical analysis for climate models.
What happened during the second week?
Figure 1: Earth energy budget comparison diagram between control (red) and flat earth (green) experiments produced by me in week 2.
In week two, with the assistance of NCAS and university scientists, we analysed climate model outputs. I personally was involved in the Flat Earth experiment- in which we tested the effect of changing surface elevation for terrain such as mountains, high plateaus on the climate. In this experiment, the perturbation is imposed by reducing the elevation of mountains to sea level. There were eight people in our team. As you may know, we PhD students have the occasional opportunity to do research collaboratively with other students in Reading Meteorology and encourage our teammates. For this reason, it felt very nice to me to work as the part of a research group. I was amazed by how we had been able to produce a small good piece of scientific work just within a matter of days due to our team effort. In Figure 1, I have presented a small part of our work which is the global energy budget comparison between a control experiment and the flat earth experiment (where the elevation of the mountains has been reduced to sea level). Along with our practical, we also attended some lectures on the ocean dynamics and physics, water in the climate system and land-atmosphere coupling and surface energy balance during this week.
How was it like to socialize with people virtually?
We used the Gather.town during the lunchtime and after work to socialize. I was a bit surprised though that I was the only student joining the Gather.town and as a result I always had to hang out (virtually) with NCAS and university scientists all the time. I rather consider it a blessing for me as there was no competition to introduce myself to the professionals. I even received a kind offer from one of our professors to assist him as a teaching assistant in his course in the department.
I learnt about some of the basic concepts of climate modelling and I hope to use these things in my research someday. It was also very refreshing to talk to and work with other students as well as the scientists. While working in a group in week 2, I once again realized there are so many things we can accomplish if we work together and encourage each other.
During the summer of 2003, Europe experienced two heatwaves with, until then, unprecedented temperatures. The 2003 summer temperature record was shattered in 2010 by the Russian heatwave, which broke even Paleo records. The question remained, if climate change influenced these two events. Many contribution studies based on the likelihood of the dynamical situation were published, providing important input to answering this question. However, the position of low and high-pressure systems and other dynamical aspects of climate change are noisy and uncertain. The storyline method attributes the thermodynamic aspects of climate change (e.g. temperature), which are visible in observations and far more certain.
All of us regularly think in terms of what ifand if only. It is the human way of calculating hypothetic results in case we would have made a different choice. This helps us think in future scenarios, trying to figure out what choice will lead to which consequence. It is a tool to reduce risk by finding a future scenario that seems the best or safest outcome. In the storyline method, we use this exact mind-set. What ifthere was no climate change, would this heatwave be the same? What if the world was 2°C warmer, what would this heatwave have looked like then? With the help of an atmospheric model we can calculate what a heatwave would have been like in a world without climate change or increased climate change.
In our study, we have two storylines: 1) the world as we know it that includes a changing climate, which we call the ‘factual’ storyline and 2) a world that could have been without climate change, which we call the ‘counterfactual’ storyline. We simulate the dynamical aspects of the weather extreme exactly the same in both storylines using a spectral nudging technique and compare the differences in temperatures. To put it more precise, the horizontal wind flow is made up out of vorticity (circular movement) and divergence (spreading out or closing in). We nudge (or push) these two variables in the higher atmosphere to, on large scale, be the same in the factual and counterfactual simulations.
Figure 1. What if we had another world where climate change did not happen? Would the heatwave have been different? Thinking in counterfactual worlds where we made (or will make) different decisions is a common way of thinking to estimate risk. Now we apply this idea in atmospheric modelling.
European 2003 and Russian 2010 heatwaves
Both the European heatwave in 2003 and the Russian heatwave in 2010 were extremes with unprecedented high temperatures for long periods of time. Besides, there had been little rain already from spring in either case, which reduced the cooling effect from moisty soil to nearly nothing. In our analysis we averaged the near surface temperatures in both storylines and compared their output to each other as well as the local climatology. Figure 2 shows the results of that averaging for the European heatwave in panel a and the Russian heatwave in panel b. We focus on the orange boxes, where the blue lines (factual storyline) and the red lines (counterfactual storyline) exceed the 5th-95th percentile climatology (green band). This means that during those days the atmosphere near the surface was uncommonly hot (thus a heatwave). The most important result in this graph is that the blue and red lines are separate from each other in the orange boxes. This means that the average temperature of the world with climate change (blue, factual) is higher than in the world without climate change (red, counterfactual).
“Even though there would have been a heatwave with or without climate change, climate change has made the heat more extreme”
Figure 2. Daily mean temperature at 2 meters height for (a) European summer 2003 and (b) Russian summer 2010. The orange boxes are the heatwaves, where the temperatures of the factual (blue) and counterfactual (red) are above the green band of 5th – 95th percentile climatology temperatures.
The difference between these temperatures are not the same everywhere, it strongly depends on where you are in Europe or Russia. Let me explain what I mean with the help of Figure 3 with the difference between factual and counterfactual temperatures (right panels) on a map. In both Europe and Russia, we see that there are local regions with temperature differences of almost 0°C, and we see regions where the differences are almost 2.5°C (for Europe) or even 4°C (for Russia). A person living south from Moscow would therefore not have experienced 33°C but 29°C in a world without climate change. It is easy to imagine that such a temperature difference changes the impacts a heatwave has on e.g. public health and agriculture.
Figure 3. Upper left: Average Temperature at 2 meter height and Geopotential height over Europe at z500 for 1-15thof August 2003, Lower left: Same as upper left but for 1-15th of Russia August 2010. Upper right: Factual minus Counterfactual average temperature at 2 meter height over Europe for 1-15th ofAugust 2003, Lower right: same as lower left but for 1-15th of Russia August 2010. Stippling indicates robust results (all factuals are >0.1°C warmer than all counterfactuals)
“The 2003 European and 2010 Russian heatwaves could locally have been 2.5°C – 4°C cooler in a world without climate change”
We can conclude therefore, that with the help of our nudged storyline method, we can study the climate signal in extreme events with larger certainty.
If you are interested in the elaborate explanation of the method and analysis of the two case studies, please take a look at our paper:
van Garderen, L., Feser, F., and Shepherd, T. G.: A methodology for attributing the role of climate change in extreme events: a global spectrally nudged storyline, Nat. Hazards Earth Syst. Sci., 21, 171–186, https://doi.org/10.5194/nhess-21-171-2021 , 2021.
This year’s Conference of the Parties (COP) should have taken place earlier in November, hosted by the UK in Glasgow and in partnership with Italy. Despite many global events successfully moving online this year, from film festivals to large conferences such as the EGU general assembly, the international climate talks were postponed until November 2021.
Unimpressed by the delay of important climate talks and negotiations, students and youth activists from FFF and a multitude of groups and movements have initiated the MOCK COP26, a 2-week online global conference on climate change that mirrors the real COP.
“My country, the Philippines, is struggling. We don’t want more floods that rise up to 15 feet, winds that peel off roofs in seconds, the rain that drowns our pets and livestock, and storm surges that ravage coastal communities. We don’t want more people to die. We’re still a developing country that contributes so little to global carbon emissions yet we face the worst of its consequences. This is absurd!
Over a dozen academic support videos break down complicated topics such as “The Kyoto Protocol”, “Agriculture and Agribusiness”, and the “History of Climate Negotiation”. These videos are helping youth delegates and all participants to understand what happens at a COP summit.
Panel sessions have featured United Nations Youth Envoy Jayathma Wickramanayake, 9 year old Climate & Environmental Activist Licypriya Kangujam, and (actual) COP26 president Alok Sharma.
High Level Country Statements
A unique aspect of MOCK COP that I have been excitedly anticipating is the high level country statements; each a 3 minute speech given by youth climate activists representing their nation.
Mock COP26 is not dominated by big polluters as COP26 is. We believe that we need to amplify the people on the frontlines of climate change, which is why we will be aiming to, throughout Mock COP, uplift the voices of those from MAPA (Most Affected People and Areas) countries above those from the Global North. This is why Mock COP26 is special.
Youth delegates have been encouraged to give speeches in whichever language they are most comfortable talking. At the time of writing, subtitles don’t appear to be fully functioning. However a large number of talks are given in English, and transcripts of all talks have been made available here: https://drive.google.com/drive/folders/1wnQUMt-rcD9XoKtg8YPWba_LZSf16qTD
I highly recommend setting some time aside to give these speeches a listen. Although the total number might put you off, it is very easy to jump in and out of talks. You can find videos embedded below, or on the official youtube channel.
Pick: Two youth delegates represent Morocco. Whilst Morocco has been ranked a role model for climate action, the reality of the country’s future is alarming. Globally the most affected are the least protected. It’s time for world leaders to protect everyone.
Pick: The delegate for Suriname explains risks faced as a Small Island Developing State (SIDS) with infrastructure near the coast. Suriname must implement climate adaptation whilst enhancing its legislation in forestry, mining, and agriculture.
Pick: Indonesia’s delegate opens with the stark warning that the country will lose 1500 of its islands due to rising sea levels by 2050. The high level statement includes calls to incorporate climate education into the national curriculum, and find ways to protect natural habitat. Indonesia has the 2nd biggest rainforest in the world, but currently has no agreed emissions reductions pathway.
Pick: Ireland’s youth delegates present a necessarily progressive 5 year plan to stick to the EU target of reducing emissions by at least 65% by 2030. The need for much stronger climate education, and providing access to affordable and sustainable energy, are among many other commitments.
Pick: The year started with forest fires devastating large swathes of Australia’s natural habitats. Youth delegates want their nation to lead the world as a renewable energy exporter, and an overhaul of media rules to foster new diverse media outlets and prevent monopolies that currently stall climate action.
But youth messages can be heard loud and clear at MOCK COP 26, reflecting the 5 themes of the conference.
We demand concrete action, not mere promises. It’s time for our leaders to wake up, prioritize the realization of the Green Deal, and cut carbon emissions.
We won’t have more time to alter the effects of the climate crisis if we let this opportunity pass. The clock is ticking. The time for action is NOW.
In the wake of covid-19 induced economic shocks, policy makers must ensure genuine green recovery that engages with ideas of global climate justice.
Youth delegate panels will continue over the weekend, working towards the creation of a final statement outlining their demands for world leaders. This will be presented to High Level Climate Action Champion for COP26 Nigel Topping, at the closing ceremony (12:00 GMT Tuesday 1st December)
To date, the 2020 North Atlantic hurricane season has been the most active on record, producing 20 named storms, 7 hurricanes, and a major hurricane which caused $9 billion in damages across the southern United States. With the potential for such destructive storms, it is understandable that a large amount of attention is paid to the North Atlantic basin at this time of year. Whilst hurricanes have been known to cause devastation in the tropics for centuries, until recently there was little appreciation for the destructive potential of these systems across Europe.
As tropical cyclones such as hurricanes move poleward – away from the tropics and into regions of lower sea surface temperatures and higher vertical wind shear, they undergo a process called extratropical transition (Klein et al., 2000): Over a period of time, the cyclones change from symmetric, warm cored systems into asymmetric cold core systems fuelled by horizontal temperature gradients, as opposed to latent heat fluxes (Evans et al., 2017). These systems, so-called post-tropical cyclones (PTCs), often reintensify in the mid-latitude Atlantic with consequences for land masses downstream – often Europe. This was highlighted in 2017, when ex-hurricane Ophelia impacted Ireland, bringing with it the strongest winds Ireland had seen in 50 years (Stewart, 2018). 3 people were killed, and 360,000 homes were without power.
In a recent paper, we quantify the risk associated with PTCs across Europe relative to mid-latitude cyclones (MLCs) for the first time – in terms of both the absolute risk (i.e. what fraction of high impact wind events across Europe are caused by PTCs?) and also the relative risk (for a given PTC, how likely is it to be associated with high-impact winds, and how does this compare to a given MLC?). By tracking all cyclones impacting a European domain (36-70N, 10W-30E) in the ERA5 reanalysis (1979-2017) using a feature tracking algorithm (Hodges, 1994, 1995, 1999), we identify the post-tropical cyclones using spatiotemporal matching (Hodges et al., 2017) with the observational record, IBTrACS (Knapp et al., 2010).
Figure 1 shows the distributions of maximum intensity for PTCs and MLCs across the entire European domain (top), Northern Europe (48-70°N, 10°W-30°E; middle) and Southern Europe (36-48°N, 10°W-30°E; bottom), using all cyclone tracks all year round. The distribution of PTC maximum intensities is higher (in terms of both wind speed and MSLP) than MLCs, particularly across Northern Europe. The difference between the maximum intensity distributions of PTCs and MLCs across Northern Europe is statistically significant (99%). PTCs are also present in the highest of intensity bins, indicating that the strongest PTCs have intensities comparable to strong wintertime MLCs.
Whilst Figure 1 shows that PTCs are stronger than MLCs even when considering MLCs forming all year round (including the often much stronger wintertime MLCs), it is also useful to compare the risks posed by PTCs relative to MLCs forming at the same time of the year – during the North Atlantic hurricane season (June 1st-November 30th).
Figure 2 shows the fraction of all storms, binned by their maximum intensity in their respective domains, which are PTCs. For storms with weak-moderate maximum winds (first three bins in the figure), <1% of such events are caused by PTCs (with the remaining 99% caused by MLCs). For stronger storms, particularly those of storm force (>25 ms-1 on the Beaufort scale), this percentage is much higher. Despite less than 1% of all storms impacting Northern Europe during hurricane season being PTCs, almost 9% of all storms with storm-force winds which impact the region are PTCs, suggesting that a disproportionate fraction of high-impact windstorms are PTCs. 8.2% of all Northern Europe impacting PTCs which form during hurricane season impact the region with storm-force winds. This fraction is only 0.8% for MLCs, suggesting that the fraction of PTCs impacting Northern Europe with storm-force winds is approximately 10 times greater than MLCs.
Here we have shown that PTCs, at their maximum intensity over Northern Europe, are stronger than MLCs. However, the question still remains as to why this is the case. Warm-seclusion storms post-extratropical transition have been shown to have the fastest rates of reintensification (Kofron et al., 2010) and typically have the lowest pressures upon impacting Europe (Dekker et al., 2018). Given the climatological track that PTCs often take over the warm waters of the Gulf stream, along with the contribution of both baroclinic instability and latent heat release for warm-seclusion development (Baatsen et al., 2015), one hypothesis may be that PTCs are more likely to develop into warm seclusion storms than the broader class of mid-latitude cyclones, potentially explaining the disproportionate impacts they cause across Europe. This will be the topic of future work.
Despite PTCs disproportionately impacting Europe with high intensities, they are a relatively small component of the total cyclone risk in the current climate. However, only small changes are expected in MLC number and intensity under RCP 4.5 (Zappa et al., 2013). Conversely, the number of hurricane-force (>32.6 ms-1) storms impacting Norway, the North Sea and the Gulf of Biscay has been projected to increases by a factor of 6.5, virtually all of which originate in the tropics (Haarsma et al., 2013). Whilst the absolute contribution of PTCs to hurricane season windstorm risk is currently low, PTCs may make an increasingly significant contribution to European windstorm risk in a future climate.
Sainsbury, E. M., R. K. H. Schiemann, K. I. Hodges, L. C. Shaffrey, A. J. Baker, K. T. Bhatia, 2020: How Important Are Post‐Tropical Cyclones for European Windstorm Risk? Geophysical Research Letters, 47(18), e2020GL089853, https://doi.org/10.1029/2020GL089853
Baatsen, M., Haarsma, R. J., Van Delden, A. J., & de Vries, H. (2015). Severe Autumn storms in future Western Europe with a warmer Atlantic Ocean. Climate Dynamics, 45, 949–964. https://doi.org/10.1007/s00382-014-2329-8
Dekker, M. M., Haarsma, R. J., Vries, H. de, Baatsen, M., & Delden, A. J. va. (2018). Characteristics and development of European cyclones with tropical origin in reanalysis data. Climate Dynamics, 50(1), 445–455. https://doi.org/10.1007/s00382-017-3619-8
Evans, C., Wood, K. M., Aberson, S. D., Archambault, H. M., Milrad, S. M., Bosart, L. F., et al. (2017). The extratropical transition of tropical cyclones. Part I: Cyclone evolution and direct impacts. Monthly Weather Review, 145(11), 4317–4344. https://doi.org/10.1175/MWR-D-17-0027.1
Haarsma, R. J., Hazeleger, W., Severijns, C., De Vries, H., Sterl, A., Bintanja, R., et al. (2013). More hurricanes to hit western Europe due to global warming. Geophysical Research Letters, 40(9), 1783–1788. https://doi.org/10.1002/grl.50360
Knapp, K. R., Kruk, M. C., Levinson, D. H., Diamond, H. J., & Neumann, C. J. (2010). The international best track archive for climate stewardship (IBTrACS). Bulletin of the American Meteorological Society, 91(3), 363–376. https://doi.org/10.1175/2009BAMS2755.1
Kofron, D. E., Ritchie, E. A., & Tyo, J. S. (2010). Determination of a consistent time for the extratropical transition of tropical cyclones. Part I: Examination of existing methods for finding “ET Time.” Monthly Weather Review, 138(12), 4328–4343. https://doi.org/10.1175/2010MWR3180.1
Stewart, S. R. (2018). Tropical Cyclone Report: Hurricane Ophelia. National Hurricane Center, (February), 1–32. https://doi.org/AL142016
Zappa, G., Shaffrey, L. C., Hodges, K. I., Sansom, P. G., & Stephenson, D. B. (2013). A multimodel assessment of future projections of North Atlantic and European extratropical cyclones in the CMIP5 climate models. Journal of Climate, 26(16), 5846–5862. https://doi.org/10.1175/JCLI-D-12-00573.1
The phrase “British summer” tends to evoke images of disorganised family barbecues being interrupted by heavy rain, or the covers coming on at Wimbledon, or the saying “three fine days and a thunderstorm”. Yet in recent years, hot weather has become an increasingly regular occurrence. Let me take you on a brief tour of notably hot summers in the UK. I’ll largely draw on the Met Office HadUK-Grid dataset, shown in Figure 1.
HadUK-Grid begins in 1884, but thanks to the Central England Temperature dataset (which extends back to 1659), we do have records of earlier heatwaves. These include the hot summer of 1666, which set the scene for the Great Fire of London in September. The summers of 1781, 1826 and 1868 were also particularly hot. The first hot summer in the HadUK-Grid series is 1899, which was the warmest summer by average maxima in that series until 1976!
But our journey properly begins in 1911, when the temperature reached 36.7°C on August 9th. At the time, this was the highest reliably recorded temperature measured in the UK. It is hard to imagine how this summer must have felt at the time – not least in the cooler average climate, but also with the less developed infrastructure and clothing customs of the time. As with any heatwave, its impacts were large with increased death, drought, and agricultural impacts. The summer of 1911 was followed by the summer of 1912, which was the 2nd wettest on record for the UK. Such a turnaround must have been equally hard to believe and does highlight that extreme swings in the British weather are not, in themselves, new. In a series from 1884, the summer of 1911 is the 8th warmest in terms of the UK average maximum temperature (at the time, it would have been 2nd, with only 1899 warmer).
Stopping briefly in 1933 (which eclipsed 1911, but pales in comparison with the dustbowl conditions being experienced in the US at the time) and then again in August 1947 (which remains 2nd warmest for UK average maxima and the nation’s driest, and was in huge contrast to the tremendously snowy and cold February), our next destination is 1975.
1975 currently ranks as the 11th warmest for UK average maxima but is also the 19th driest. This, when combined with the dry winter that followed, set the scene for the infamous summer of 1976. Both these summers followed a spell of very cool summers, with no particularly remarkable summers in the 1960s, while the UK did not see a temperature above 28°C in 1974 (almost unthinkable nowadays). I won’t go into huge detail about the 1976 summer, but it is engrained in the minds of a generation thanks not only to its remarkable June heatwave (which has never been matched) but also the cool climate in which it occurred. It ranks as the 2nd driest summer for the UK and remains the warmest on record in terms of average maxima – though no individual month holds the number 1 spot.
Let us next whizz off to July 1983, which at the time had the warmest nationally averaged maxima for the month (it now ranks 3rd). Oddly enough, while the UK baked in heat, the temperature at Vostok, Antarctica dropped to -89.2°C on the 21st – the lowest surface-based temperature ever recorded. I am keeping the topic of this blog to hot summers, but I want to give 1985 a special mention – the most recent summer when the UK-average maxima were less than 17°C, a formerly frequent occurrence.
As we hot-foot it toward the end of the 20th century (pun intended), we arrive at 1990. Liverpool had just won the First Division (sound familiar?) and on August 3rd the temperature at Cheltenham, Gloucestershire reached 37.1°C – beating the record set in 1911 after 79 years. That night, the temperature fell to only 23.9°C in Brighton – the warmest night on record. However, the heatwave was rather brief but intense (3 consecutive days exceeded 35°C, the only other occurrences were in 1976). For a prolonged heatwave, we jump to August 1995. With a UK average maximum of 22.8°C, it remains the UK’s warmest August by that metric, and the 2nd driest. The summer ranks 2nd warmest by maxima. Soon after, the August of 1997 (4th warmest) added to growing evidence of a change to the British climate.
But it was in the August of 2003 when things really kicked off. In the earliest heatwave I remember, the temperature hit 38.5°C on the 10th at Faversham, Kent (satellite image in Figure 2) – the first time the UK had surpassed 37.8°C (100°F) and breaking the record from 1990 after only 23 years. 30°C was exceeded somewhere for 10 consecutive days. The summer of 2003 ranks nowadays as 6th warmest by average maxima; across Europe conditions were more extreme with a huge estimated death toll.
Only 3 years later, July 2006 set the record for the hottest month for the UK-average maxima (23.3°C), and set – at the time – a record for the highest-recorded July temperature (36.5°C at Wisley on the 19th). Ranking 4th warmest by average maxima, the summer was even more extreme across mainland Europe.
What followed from 2007 through 2012 was a spell of wet summers, but we shrug off all that Glastonbury mud to arrive at July 2013, which currently ranks as 4th warmest by average maxima and saw the longest spell of >28°C weather since 1997.
Skipping through in increasingly short steps, we arrive for a brief blast on July 1st, 2015 – when the July record from 2006 fell, with 36.7°C at Heathrow in an otherwise cool month. We hop over now to 2018…
The summer of 2018, memorable for England’s performance in the World Cup, saw very warm temperatures in June and July. By nationally averaged maxima, June 2018 ranks 2nd behind 1940, and July sits 2nd behind 2006. The summer ranks 3rd, but by mean temperature is the warmest. Though not reaching the dizzying highs of 2003 (“only” 35.3°C was reached on July 26th), the prolonged dry conditions which began in May across England led to parched grasses (Figure 3), wildfires, and low river levels. I may have also had a viral tweet.
With the present day in sight, our journey is not yet over. Stepping into 2019, an otherwise unremarkable summer was characterised with huge bursts of heat – setting records across Europe – which on July 25th saw the temperature reach 38.7°C at Cambridge Botanic Gardens. This eclipsed the 2003 record and became only the 2nd day – at the time – when 100°F or more had been reached in the UK.
But that is still not the end of the story! After a record-setting sunny spring followed by a mixed first half of summer, on July 31st 2020 the temperature at Heathrow hit 37.8°C – becoming the UK’s third warmest day on record and the third time 100°F had been recorded. The following Friday, 36.4°C was reached at Heathrow and Kew – the UK’s 9th warmest day on record, and highest temperature in August since 2003. Figure 4 shows the view at the University atmospheric observatory shortly after 34.8°C was reached, Reading’s 4th highest in August since records began in 1908.
Forecasts suggest a continuation of hot weather through the next week or so, with many records up for grabs. However, we should be mindful that heatwaves cause suffering and excess deaths, too. And, with the evidently increasing frequency with which these hot extremes are occurring (note how so many of the stops on my tour were clustered in the last 30 years), they are not good news, but another sign that our climate is changing.
Now that we have blasted through the 100°F barrier, our attention turns to 40°C. Research suggests this is already becoming much more likely thanks to climate change and will continue to do so. Reaching such extremes in the UK requires a unique combination of factors – but when these do come together, expect yet more records to fall.
Downward trends in Arctic sea-ice extent in recent decades are a striking signal of our warming planet. Loss of sea ice has major implications for future climate because it strongly influences the Earth’s energy budget and plays a dynamic role in the atmosphere and ocean circulation.
Comprehensive numerical models are used to make long-term projections of the future climate state under different greenhouse gas emission scenarios. They estimate that the Arctic ocean will become seasonally ice free by the end of the 21st century, but there is a large uncertainty on the timing due to the spread of estimates across models (Fig. 1).
What causes this spread, and how might it be reduced to better constrain future projections? There are various factors (Notz et al. 2016), but of interest to our work is the large-scale forcing of the atmosphere and ocean. The mean atmospheric circulation transports about 3 PW of heat from lower latitudes into the Arctic, and the oceans transport about a tenth of that (e.g. Trenberth and Fasullo, 2017; 1 PW = 1015 W). Our goal is to understand the relative roles of Ocean and Atmospheric Heat Transports (OHT, AHT) on long timescales. Specifically, how sensitive is the sea-ice cover to deviations in OHT and AHT, and what underlying mechanisms determine the sensitivities?
We developed a highly simplified Energy-Balance Model (EBM) of the climate system (Fig. 2)—it has only latitudinal variations and is described by a few simple equations relating energy transfer between the atmosphere, ocean, and sea ice (Aylmer et al. 2020). The latitude of the sea-ice edge is an analogue for ice extent in the real world. The simplicity of the EBM allows us to isolate the basic physics of the problem, which would not be possible going directly with the complex output of a full climate model.
We generated a set of simulations in which OHT varies and checked the response of the ice edge. This is a measure of the effective sensitivity of the ice cover to OHT (Fig. 3a)—it is not the actual sensitivity because AHT decreases (Fig. 3b), and we are really seeing in Fig. 3a the net response of the ice edge to changes in both OHT and AHT.
This reduction in AHT with increasing OHT is called Bjerknes compensation, and it occurs in full climate models too (Outten et al. 2018). Here, it has a moderating effect on the true impact of increasing OHT. With further analysis, we determined the actual sensitivity to be about 1.5 times the effective sensitivity. The actual sensitivity of the ice edge to AHT turns out to be about half that to the OHT.
What sets the difference in OHT and AHT sensitivities? This is easily answered within the EBM framework. We derived a general expression for the ratio of (actual) ice-edge sensitivities to OHT (so) and AHT (sa):
A higher-order term has been neglected for simplicity here, but the basic point remains: the ratio of sensitivities mainly depends on the parameters BOLR and Bdown. These are bulk representations of atmospheric feedbacks and determine the efficiency of outgoing and downwelling longwave radiation, respectively. They are always positive, so the ice edge is always more sensitive to OHT than AHT.
The interpretation of this equation is simple. AHT converging over the ice pack can either be transferred to the underlying sea ice, or radiated to space, having no impact on the ice, and the partitioning is controlled by Bdown and BOLR. The same amount of OHT converging under the ice pack can only go through the ice and is thus the more efficient driver.
Climate models with larger OHTs tend to have less sea ice (Mahlstein and Knutti, 2011). We have also found strong correlations between OHT and the sea-ice edge in several of the models listed in Fig. 1 individually. Ice-edge sensitivities and B values can be determined per model, and our equation predicts how these should be related. Our work thus provides a way to investigate how much physical biases in OHT and AHT contribute to sea-ice-projection uncertainties.
Methane (CH4) is a potent greenhouse gas. Its ability to effectively alter fluxes of longwave (thermal-infrared) radiation emitted and absorbed by the Earth’s surface and atmosphere has been well studied. As a result, methane’s thermal-infrared impact on the climate system has been quantified in detail. According to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), methane has the second largest radiative forcing (0.48 W m-2) of the well-mixed greenhouse gases after carbon dioxide (CO2) (Myhre et al. 2013, See Figure 1). This means that due to its change in atmospheric concentration since the pre-industrial era (from 1750 – 2011), methane has directly perturbed the tropopause net (incoming minus outgoing) stratospheric temperature-adjusted radiative flux by 0.48 W m-2, causing the climate system to warm.
However, an important effect is missing from the current IPCC AR5 estimate of methane’s climate impact – its absorption of solar radiation. In addition to longwave radiation, methane also alters fluxes of incoming solar shortwave radiation at wavelengths between 0.7 – 10 µm.
Until recently this shortwave effect had not been thoroughly quantified and as such was largely overlooked. My PhD work focuses on quantifying methane’s shortwave effect in detail and aims to build upon the significant, initial findings of Etminan et al. (2016) and a more recent study by Collins et al. (2018).
Etminan et al. (2016) analysed methane’s absorption of solar near-infrared radiation (at wavelengths between 0.2 – 5 µm) and found that it exerted a direct instantaneous, positive forcing on the climate system, largely due to the absorption of upward scattered, reflected near-infrared radiation by clouds in the troposphere. Essentially, this processes results in photons taking multiple passes throughout the troposphere, which in turn results in increased absorption by CH4. Figure 2 shows the net (downwards minus upwards) spectral variation of this forcing at the tropopause under all-sky (i.e. cloudy) conditions. Here it is clear to see methane’s three key absorption bands across the near-infrared region at 1.7, 2.3 and 3.3 µm.
As Etminan et al. (2016) explain, following a perturbation in methane concentrations all of these bands decrease the downwards shortwave radiative flux at the tropopause, due to increased absorption in the stratosphere. However, the net sign of the forcing depends on whether this negative contribution compensates over increased absorption by these bands in the troposphere (which constitutes a positive forcing). As Figure 2 shows, whilst the 3.3 µm band has a strongly net negative forcing due to the absorption of downwelling solar radiation in the stratosphere, both the 1.7 µm and 2.3 µm bands have a net positive forcing due to increased CH4 absorption in an all-sky troposphere. When summed across the entire spectral range, the positive forcing at 1.7 µm and 2.3 µm dominates over the negative forcing at 3.3 µm – resulting in a net positive forcing. Etminan et al. (2016) also found that the nature of this positive forcing is partly explained by methane’s spectral overlap with water vapour (H2O). The 3.3 µm band overlaps with a region of relatively strong H2O absorption, which reduces its ability to absorb shortwave radiation in the troposphere, where high concentrations of H2O are present. However, both the 1.7 µm and 2.3 µm bands overlap much less with H2O, and so are able to absorb more shortwave radiation in the troposphere.
In addition to this, Etminan et al. (2016) also found that the shortwave effect serves to impact methane’s stratospheric temperature-adjusted longwave radiative forcing (the process whereby stratospheric temperatures readjust to radiative equilibrium before the change in net radiative flux is calculated at the tropopause; Myhre et al. (2013)). Here, absorption of solar radiation in the stratosphere results in a warmer stratosphere, and hence increased emission of longwave radiation by methane downwards to the troposphere. This process results in a positive tropopause longwave radiative forcing. Combing both the direct, instantaneous shortwave forcing and its impact on the stratospheric-temperature adjusted longwave forcing, Etminan et al. (2016) found that the inclusion of the shortwave effect enhances methane’s radiative forcing by a total of 15%. The results presented in this study are significant and indicate the importance of methane’s shortwave absorption. However, Etminan et al. (2016) note several areas of uncertainty surrounding their estimate and highlight the need for a more detailed analysis of the subject.
My work aims to address these uncertainties by investigating the impact of factors such as updates to the HITRAN spectroscopic database (which provides key spectroscopic parameters for climate models to simulate the transmission of radiation through the atmosphere), the inclusion of the solar mid-infrared (7.7 µm) band in calculations of the shortwave effect and potential sensitivities, such as the vertical representation of CH4 concentrations throughout the atmosphere and the specification of land surface albedo. My work also extends Etminan et al. (2016) by investigating the shortwave effect at a global spatial resolution, since a two-atmosphere approach (using tropical and extra-tropical profiles) was employed by the latter. To do this I use the model SOCRATES-RF (Checa-Garcia et al. 2018) which computes monthly-mean radiative forcings at a global 5° x 5° spatial resolution using a high resolution 260-band shortwave spectrum (from 0.2 – 10 µm) and a standard 9-band longwave spectrum.
Initial results calculated by SOCRATES-RF confirm that methane’s all-sky tropopause shortwave radiative forcing is positive and that the inclusion of shortwave bands serves to increase the stratospheric-temperature adjusted longwave radiative forcing. In total my calculations estimate that the shortwave effect increases methane’s total radiative forcing by 10%. Whilst this estimate is lower than the 15% proposed by Etminan et al. (2016) it’s important to point out that this SOCRATES-RF estimate is not a final figure and investigations into several key forcing sensitivities are currently underway. For example, methane’s shortwave forcing is highly sensitive to the vertical representation of concentrations throughout the atmosphere. Tests conducted using SOCRATES-RF reveal that when vertically-varying profiles of CH4 concentrations are perturbed, the shortwave forcing almost doubles in magnitude (from 0.014 W m-2 to 0.027 W m-2) when compared to the same calculation conducted using constant vertical profiles of CH4 concentrations. Since observational studies show that concentrations of methane decrease with height above the tropopause (e.g. Koo et al. 2017), the use of realistic vertically-varying profiles in forcing calculations are essential. Highly idealised vertically-varying CH4 profiles are currently employed in SOCRATES-RF, which vary with latitude but not with season. Therefore, the realism of this representation needs to be validated against observational datasets and possibly updated accordingly.
Another key sensitivity currently under investigation is the specification of land surface albedo – a potentially important factor controlling the amount of reflected shortwave radiation absorbed by methane. Since the radiative properties of surface features are highly wavelength-dependent, it is plausible that realistic, spectrally-varying land surface albedo values will be required to accurately simulate methane’s shortwave forcing. For example, vegetation and soils typically tend to reflect much more strongly in the near-infrared than in the visible region of the solar spectrum, whilst snow surfaces reflect much more strongly in the visible (see Roesch et al. 2002). Currently in SOCRATES-RF, globally-varying, spectrally-constant land-surface albedo values are used, derived from ERA-Interim reanalysis data.
Figure 3 compares the spatial distribution of methane’s annual-mean all-sky tropopause shortwave forcing as calculated by SOCRATES-RF and Collins et al. (2018). Both calculations exhibit the same regions of maxima across, for example, the Sahara, the Arabian Peninsula, and the Tibetan Plateau. However, it is interesting to note that the poleward amplification shown by SOCRATES-RF is not evident in Collins et al. (2018). The current leading hypothesis for this difference is the fact that the land surface albedo is specified differently in each calculation. Collins et al. (2018) employ spectrally-varying surface albedo values derived from satellite observations. These are arguably more realistic than the spectrally-constant values currently specified in SOCRATES-RF. The next step in my PhD is to further explore the interdependence between methane’s shortwave forcing and land-surface albedo, and to work towards implementing spectrally-varying albedo values into SOCRATES-RF calculations. Along with the ongoing investigation into the vertical representation of CH4 concentrations, I aim to finally deliver a more definitive estimate of methane’s shortwave effect.
Checa-Garcia, R., Hegglin, M. I., Kinnison, D., Plummer, D. A., and Shine, K. P. 2018: Historical tropospheric and stratospheric ozone radiative forcing using the CMIP6 database. Geophys. Res. Lett., 45, 3264–3273, https://doi.org/10.1002/2017GL076770
Etminan, M., G. Myhre, E. Highwood, K. P. Shine. 2016: Radiative forcing of carbon dioxide, methane and nitrous oxide: a significant revision of methane radiative forcing, Geophys. Res. Lett., 43, https://doi.org/10.1002/2016/GL071930
IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 1535 pp
Myhre, G., et al. 2013: Anthropogenic and natural radiative forcing, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by T. F. Stocker et al., pp. 659–740, Cambridge Univ. Press, Cambridge, U. K., and New York.
Roesch, A., M. Wild, R. Pinker, and A. Ohmura, 2002: Comparison of spectral surface albedos and their impact on the general circulation model simulated surface climate, J. Geophys. Res., 107, 13-1 – 13-18, https://doi.org/10.1029/2001JD000809
While some may consider the philosophy of science a complicating distraction, I think I ignore it at my peril. Certainly climate science is not without its philosophical issues; one might even say it is riddled with them…
David Stainforth (LSE), the keynote speaker stated it thus: “The study of anthropogenic climate change presents a range of fundamental challenges for scientific and wider academic inquiry. The essential nature of these challenges are often not well appreciated.”
So how does climate science compare with other natural sciences? Opinions abounded, but here are just some I can recall: 1 – We can’t really conduct controlled experiments in the way that other natural scientists can, as we have just the one Earth and can’t turn back time (we have to beware of post-hoc explanations, and some of our predictions may never be verifiable/falsifiable). 2 – We therefore rely heavily on numerical models. 3 – We’re also doing our science while the climate is changing around us, and thus there is a strong sense of urgency. 4 – There is therefore a pressure to be multidisciplinary.
On a more practical side, David’s talk left me with a novel way of thinking about ‘climate’. Thinking about a climate metric, such as temperature, I would have thought hitherto of simply a mean and a standard deviation (a very Gaussian way of looking at it!). But David argued that climate is often best conceived of as a more generalised distribution. While a bell curve is symmetric, unimodal, a distribution need not be (and this can be true in the climate system). Studying and predicting a stable climate distribution may already be difficult but studying and predicting a changing one is even harder!
Now for a bit of a whirlwind tour of other arguments/points. There was Reading’s own Ted Shepherd arguing that in climate science we often over focus on avoiding false positives (type I errors) at the expense of incurring false negatives (type II). In other words, we get reliability at the price of informativeness, especially at the regional level where policy makers are somewhat eager to be informed.
Then there was Geoff Vallis (University of Exeter) who posed the question “If models were perfect, would we care how they worked?”. Perhaps a pertinent question, as there appears to be a trade-off, an inverse correlation between complexity of models and our ability to understand them. If the models became so complex that they were beyond the abilities of any human past or future to comprehend, what would we do then? If they become as complicated as the Earth system itself, surely we would have long since lost any grasp on them? Indeed, models already appear to be predicting phenomena without us understanding why. Complexity is not necessarily accuracy (How do we assess accuracy in climate science?) and Erica Thompson (LSE) highlighted the importance of ‘getting out of model land’, and staying with the real world, something some of us may need occasional reminding of.
What even are models? Two expressions given were ‘book-keeping devices’ (Wendy Parker) and ‘Prosthesis of your brain (Erica Thompson). No doubt there were others.
Marina Baldissera Pacchetti (University of Leeds) talked about her work on climate information for adaptation that gives us: “guidelines on when quantitative statements about future climate are warranted and potentially decision-relevant, when these statements would be more valuable taking other forms (for example, qualitative statements), and when statements about future climate are not warranted at all.”
In the afternoon, there were breakout ‘lightning’ discussions. We could choose to join 1 of the following 8 groups:
1. Should we aim to estimate the mean/expectation behaviour of the climate or focus on the worst-case? 2. Is the way we go about climate science now the only way of doing it? 3. If our computers were infinitely fast, what science would we do with them? 4. If our models were infinitely good, what science would be left to do? 5. What fact, if only we knew it, would have the biggest impact on climate change? 6. How should climate science approach the question of geoengineering? 7. What is the benefit to society of general circulation models? 8. What is the public needing to know, and are we working enough on these questions?
My group was 3, but we ended up accidentally merging with 4 and made for a very interesting and varied discussion!
Which group would you have been most pulled towards had you been there? What philosophical thoughts on climate science have you had? What do you think is the most under-appreciated? I would be interested to hear your thoughts.
Many thanks to the event organiser Goodwin Gibbins (Imperial) and all involved for a thoroughly enjoyable and stimulating day.
If anyone would like to get more into the Philosophy of Science, I would recommend this thoroughly engaging 10-hour course of lecture by the Uni of Toronto on YouTube, the trailer of which can be viewed here.