Dying Coral Reefs

Global warming has been causing the "bleaching" of coral reefs. A bleached coral reef has lost its photosynthesizing symbiotic organisms, called zooxanthellae. It may look white as a ghost — as in the picture above — but it is not yet dead. If the zooxanthellae come back, the reef can recover.

With this year’s record high temperatures, many coral reefs are actually dying:

• Dan Charles, Massive coral die-off reported in Indonesia, Morning Edition, August 17, 2010.

DAN CHARLES: This past spring and early summer, the Andaman Sea, off the coast of Sumatra, was three, five, even seven degrees [Fahrenheit] warmer than normal. That can be dangerous to coral, so scientists from the Wildlife Conservation Society went out to the reefs to take a look. At that time, about 60 percent of the coral had turned white – it was under extreme stress but still alive.

Caleb McClennen from the Wildlife Conservation Society says they just went out to take a look again.

DR. CALEB MCCLENNEN: The shocking situation, now, is that about 80 percent of those that were bleached have now died.

CHARLES: That’s just in the area McClennen’s colleagues were able to survey. They’re asking other scientists to check on coral in other areas of the Andaman Sea.

Similar mass bleaching events have been observed this year in Sri Lanka, Thailand, Malaysia, and other parts of Indonesia.

For more, see:

• Environmental news service, Corals bleached and dying in overheated south Asian waters, August 16, 2010.

It’s interesting to look back back at the history of corals — click for a bigger view:

Corals have been around for a long time. But the corals we see now are completely different from those that ruled the seas before the Permian-Triassic extinction event 250 million years ago. Those earlier corals, in turn, are completely different from those that dominated before the Ordovician began around 490 million years ago. A major group of corals called the Heliolitida died out in the Late Devonian extinction. And so on.

Why? Corals live near the surface of the ocean and are thus particularly sensitive not only to temperature changes but also changes in sea levels and changes in the amount of dissolved CO₂, which makes seawater more acid.

We are now starting to see what the Holocene extinction will do to corals. Not only the warming but also the acidification of oceans are hurting them. Indeed, seawater is reaching the point where aragonite, the mineral from which corals are made, becomes more soluble in water.

This paper reviews the issue:

• O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi and M. E. Hatziolos, Coral reefs under rapid climate change and ocean acidification, Science 318 (14 December 2007), 1737-1742.

Chris Colose has a nice summary of what this paper predicts under three scenarios:

1) If CO₂ is stablilized today, at 380 ppm-like conditions, corals will change a bit but areas will remain coral dominated. Hoegh-Guldberg et al. emphasize the importance of solving regional problems such as fishing pressure, and air/water quality which are human-induced but not directly linked to climate change/ocean acidification.

2) Increases of CO₂ at 450 to 500 ppmv at current >1 ppmv/yr scenario will cause significant declines in coral populations. Natural adaptive shifts to symbionts with a +2°C resistance may delay the demise of some reefs, and this will differ by area. Carbonate-ion concentrations will drop below the 200 µmol kg^-1 threshold and coral erosion will outweigh calcification, with significant impacts on marine biodiversity.

3) In the words of the study, a scenario of >500 ppmv and +2°C sea surface temperatures “will reduce coral reef ecosystems to crumbling frameworks with few calcareous corals”. Due to latitudinally decreasing aragonite concentrations and projected atmospheric CO₂ increases adaptation to higher latitudes with areas of more thermal tolerance is unlikely. Coral reefs exist within a narrow band of temperature, light, and aragonite saturation states, and expected rises in SST’s will produce many changes on timescales of decades to centuries (Hoegh-Guldberg 2005). Rising sea levels may also harm reefs which necessitate shallow water conditions. Under business-as-usual to higher range scenarios used by the IPCC, corals will become rare in the tropics, and have huge impacts on biodiversity and the ecosystem services they provide.

The chemistry of coral is actually quite subtle. Here’s a nice introduction, at least for people who aren’t scared by section headings like “Why don’t corals simply pump more protons?”:

• Anne L. Cohen and Michael Holcomb, Why corals care about ocean acidification: uncovering the mechanism, Oceanography 22 (2009), 118-127.

Introduction to Climate Change

No, this post is not an introduction to climate change. It’s a question from Alex Hoffnung, who recently got his Ph.D. from U.C. Riverside after working with me on categorified Hecke algebras. Now he’s headed for a postdoc at the University of Ottawa. He’s a cool dude:

And he has a question that I’m sure many mathematicians and other scientists share, so I’ll make it a guest post here:

---

Have you come across anything like “Intro to Climate Change”? The big problem is that I have following the issues surrounding climate change are getting a handle on what the issues are. How hard is it to objectively state some of the more important foundational issues without running into controversy?

How Hot Is Too Hot?

How hot is too hot? This interesting paper tackles that question:

• Steven C. Sherwood and Matthew Huber, An adaptability limit to climate change due to heat stress, Proceedings of the National Academy of Sciences, early edition 2010.

Abstract: Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wetbulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.

Huh? Temperatures going up by 12 degrees Celsius??? Well, this is a worst-case scenario — the sort of thing that’s only likely to kick in if we keep up ‘business as usual’ for a long, long time:

Recent studies have highlighted the possibility of large global warmings in the absence of strong mitigation measures, for example the possibility of over 7 °C of warming this century alone. Warming will not stop in 2100 if emissions continue. Each doubling of carbon dioxide is expected to produce 1.9–4.5 °C of warming at equilibrium, but this is poorly constrained on the high side and according to one new estimate has a 5% chance of exceeding 7.1 °C per doubling. Because combustion of all available fossil fuels could produce 2.75 doublings of CO₂ by 2300, even a 4.5 °C sensitivity could eventually produce 12 °C of warming. Degassing of various natural stores of methane and/or CO₂ in a warmer climate could increase warming further. Thus while central estimates of business-as-usual warming by 2100 are 3–4 °C, eventual warmings of 10 °C are quite feasible and even 20 °C is theoretically possible.

A key notion in Sherwood and Huber’s paper is the concept of wet-bulb temperature. Apparently this term has several meanings, but Sherwood and Huber use it to mean “the temperature as measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it”.

This can be lower than the ‘dry-bulb temperature’, thanks to evaporative cooling. And that’s important, because we sweat to stay cool.

Indeed, this is the big difference between Riverside California (my permanent home) and Singapore (where I’m living now). It’s dry there, and humid here, so my sweat doesn’t evaporate so nicely here — so the wet-bulb temperature tends to be higher. In Riverside air conditioning seems like a bit of an indulgence much of the time, though it’s quite common for shops to let it run blasting until the air is downright frigid. In Singapore I’m afraid I really like it, though when I’m in control, I keep it set at 28 °C — perhaps more for dehumidification than cooling?

Sherwood and Huber write:

A resting human body generates ∼100 W of metabolic heat that (in addition to any absorbed solar heating) must be carried away via a combination of heat conduction, evaporative cooling, and net infrared radiative cooling. Net conductive and evaporative cooling can occur only if an object is warmer than the environmental wet-bulb temperature T_W, measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it. The second law of thermodynamics does not allow an object to lose heat to an environment whose T_W exceeds the object’s temperature, no matter how wet or well-ventilated. Infrared radiation under conditions of interest here will usually produce a small additional heating.

[…]

Humans maintain a core body temperature near 37 °C that varies slightly among individuals but does not adapt to local climate. Human skin temperature is strongly regulated at 35 °C or below under normal conditions, because the skin must be cooler than body core in order for metabolic heat to be conducted to the skin. Sustained skin temperatures above 35 °C imply elevated core body temperatures (hyperthermia), which reach lethal values (42–43 °C) for skin temperatures of 37–38 °C even for acclimated and fit individuals. We would thus expect sufficiently long periods of T_W > 35 °C to be intolerable.

Now, temperatures of 35 °C (we say 95 degrees Fahrenheit) are entirely routine during the day in Riverside. Of course, it’s much cooler in my un-air-conditioned home because we leave open the windows when it gets cool at night, and the concrete slab under the floor stays cool, and the house has great insulation. Still, after a few years of getting acclimated, walking around in 35 °C weather seems like no big deal. We only think it’s seriously hot when it reaches 40 °C.

But these are not wet-bulb temperatures: the humidity is usually really low! So what’s the wet-bulb temperature when it’s 35 °C and the relative humidity is, say, 20%? I should look it up… but maybe you know where to look?

If you look on page 2 of Sherwood and Huber’s paper you’ll see three graphs. The top graph is the world today. You’ll see histograms of the average temperature (in black), the average annual maximum temperature (in blue), and the average annual maximum wet-bulb temperature (in red). The interesting thing is how the red curve is sharply peaked between 15 °C and 30 °C, dropping off sharply above 31 °C.

The bottom graph shows an imagined world that’s about 12 °C warmer. It’s too hot.

As the authors note:

The highest instantaneous T_W anywhere on Earth today is about 30 °C (with a tiny fraction of values reaching 31 °C). The most-common T_{W, max} is 26–27 °C, only a few degrees lower. Thus, peak potential heat stress is surprisingly similar across many regions on Earth. Even though the hottest temperatures occur in subtropical deserts, relative humidity there is so low that T_{W, max} is no higher than in the deep tropics. Likewise, humid mid-latitude regions such as the Eastern United States, China, southern Brazil, and Argentina experience T_{W, max} during summer heat waves comparable to tropical ones, even though annual mean temperatures are significantly lower. The highest values of T in any given region also tend to coincide with low relative humidity.

But what if it gets a lot hotter?

Could humans survive > 35 °C? Periods of net heat storage can be endured, though only for a few hours, and with ample time needed for recovery. Unfortunately, observed extreme-T_W events (T_W 26 °C) are long-lived: Adjacent nighttime minima of T_W are typically within 2–3 °C of the daytime peak, and adjacent daily maxima are typically within 1 °C. Conditions would thus prove intolerable if the peak T_W exceeded, by more than 1–2 °C, the highest value that could be sustained for at least a full day. Furthermore, heat dissipation would be very inefficient unless T_W were at least 1–2 °C below skin temperature, so to sustain heat loss without dangerously elevated body temperature would require T_W of 34 °C or lower. Taking both of these factors into account, we estimate that the survivability limit for peak six-hourly T_W is probably close to 35 °C for humans, though this could be a degree or two off. Similar limits would apply to other mammals but at various thresholds depending on their core body temperature and mass.

I find the statement “Adjacent nighttime minima of T_W are typically within 2–3 °C of the daytime peak” quite puzzling. Maybe it’s true in extremely humid climates, but in dry climates it tends to cool down significantly at night. Even here in Singapore there seems to be typically a 5 °C difference between day and night. But maybe it’s less during a heat wave.

The paper does not discuss behavioral adaptations, and that makes it a bit misleading. Even without fossil fuels people can do things like living underground during the day and using windcatchers to bring cool underground air into the house. Here’s a windcatcher that my friend Greg Egan photographed in Yazd during his trip to Iran:

But, of course, this sort of world would support far fewer people than live here now!

Another obvious doubt concerns the distant past, when it was a lot warmer than now. I’m talking about the Paleogene, which ended 23 million years ago. If you haven’t heard of the Paleogene — which is term that came into play after I learned my geological time periods back in grade school — maybe you’ll be interested to hear that it’s the beginning of the Cenozoic, consisting of the Paleocene, Eocene, and Oligocene. Since then the Earth has been in a cooling phase:

How did mammals manage back then?

Mammals have survived past warm climates; does this contradict our conclusions? The last time temperatures approached values considered here is the Paleogene, when global-mean temperature was perhaps 10 °C and tropical temperature perhaps 5–6 °C warmer than modern, implying T_W of up to 36 °C with a most-common T_{W, max} of 32–33 °C. This would still leave room for the survival of mammals in most locations, especially if their core body temperatures were near the high end of those of today’s mammals (near 39 °C). Transient temperature spikes, such as during the PETM or Paleocene-Eocene Thermal Maximum, might imply intolerable conditions over much broader areas, but tropical terrestrial mammalian records are too sparse to directly test this. We thus find no inconsistency with our conclusions, but this should be revisited when more evidence is available.

Climate Stabilization Targets

I thank Walter Blackstock at the Institute of Molecular & Cell Biology here in Singapore for pointing this out:

The most distinguished group of scientists in the United States has released an important report on climate change. You can get the whole thing for free, here:

• National Research Council, National Academy of Science, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia, 2010.

But here’s the executive summary, for you executives too busy to read the whole thing. It’s clearly written, short, and earth-shakingly important. I’ve put a few key passages in boldface.

EXECUTIVE SUMMARY

Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch where human activities will largely determine the evolution of Earth’s climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia. Policy choices can be informed by recent advances in climate science that quantify the relationships between increases in carbon dioxide and global warming, related climate changes, and resulting impacts, such as changes in streamflow, wildfires, crop productivity, extreme hot summers, and sea level rise.

Since the beginning of the industrial revolution, concentrations of greenhouse gases from human activities have risen substantially. Evidence now shows that the increases in these gases very likely (>90 percent chance) account for most of the Earth’s warming over the past 50 years. Carbon dioxide is the greenhouse gas produced in the largest quantities, accounting for more than half of the current impact on Earth’s climate. Its atmospheric concentration has risen about 35% since 1750 and is now at about 390 parts per million by volume, the highest level in at least 800,000 years. Depending on emissions rates, carbon dioxide concentrations could double or nearly triple from today’s level by the end of the century, greatly amplifying future human impacts on climate.

Society is beginning to make important choices regarding future greenhouse gas emissions. One way to inform these choices is to consider the projected climate changes and impacts that would occur if greenhouse gases in the atmosphere were stabilized at a particular concentration level. The information needed to understand such targets is multifaceted: how do emissions affect global atmospheric concentrations and in turn global warming and its impacts?

This report quantifies, insofar as possible, the outcomes of different stabilization targets for greenhouse gas concentrations using analyses and information drawn from the scientific literature. It does not recommend or justify any particular stabilization target. It does provide important scientific insights about the relationships among emissions, greenhouse gas concentrations, temperatures, and impacts.

CLIMATE CHANGE DUE TO CARBON DIOXIDE WILL PERSIST MANY CENTURIES

Carbon dioxide flows into and out of the ocean and biosphere in the natural breathing of the planet, but the uptake of added human emissions depends on the net change between flows, occurring over decades to millennia. This means that climate changes caused by carbon dioxide are expected to persist for many centuries even if emissions were to be halted at any point in time.

Such extreme persistence is unique to carbon dioxide among major agents that warm the planet. Choices regarding emissions of other warming agents, such as methane, black carbon on ice/snow, and aerosols, can affect global warming over coming decades but have little effect on longer-term warming of the Earth over centuries and millennia. Thus, long-term effects are primarily controlled by carbon dioxide.

The report concludes that the world is entering a new geologic epoch, sometimes called the Anthropocene, in which human activities will largely control the evolution of Earth’s environment. Carbon emissions during this century will essentially determine the magnitude of eventual impacts and whether the Anthropocene is a short-term, relatively minor change from the current climate or an extreme deviation that lasts thousands of years. The higher the total, or cumulative, carbon dioxide emitted and the resulting atmospheric concentration, the higher the peak warming that will be experienced and the longer the duration of that warming. Duration is critical; longer warming periods allow more time for key, but slow, components of the Earth system to act as amplifiers of impacts, for example, warming of the deep ocean that releases carbon stored in deep-sea sediments. Warming sustained over thousands of years could lead to even bigger impacts.

IMPACTS CAN BE LINKED TO GLOBAL MEAN TEMPERATURES

To date, climate stabilization goals have been most often discussed in terms of stabilizing atmospheric concentrations of carbon dioxide (e.g., 350 ppmv, 450 ppmv, etc.). This report concludes that, for a variety of conceptual and practical reasons, it is more effective to assess climate stabilization goals by using global mean temperature change as the primary metric. Global temperature change can in turn be linked both to concentrations of atmospheric carbon dioxide and to accumulated carbon emissions.

An important reason for using warming as a reference is that scientific research suggests many key impacts can be quantified for given temperature increases. This is done by scaling local to global warming and by “coupled linkages” that show how other climate changes, such as alterations in the water cycle, scale with temperature. There is now increased confidence in how global warming levels of 1°C, 2°C, 3°C etc. would relate to certain future impacts. This report lists some of these effects per degree (°C) of global warming, including:

• 5-10 percent changes in precipitation in a number of regions

• 3-10 percent increases in heavy rainfall

• 5-15 percent yield reductions of a number of crops

• 5-10 percent changes in streamflow in many river basins worldwide

• About 15 percent and 25 percent decreases in the extent of annually averaged and September Arctic sea ice, respectively

For warming of 2°C to 3°C, summers that are among the warmest recorded or the warmest experienced in people’s lifetimes, would become frequent. For warming levels of 1°C to 2°C, the area burned by wildfire in parts of western North America is expected to increase by 2 to 4 times for each degree (°C) of global warming. Many other important impacts of climate change are difficult to quantify for a given change in global average temperature, in part because temperature is not the only driver of change for some impacts; multiple environmental and other human factors come into play. It is clear from scientific studies, however, that a number of projected impacts scale approximately with temperature. Examples include shifts in the range and abundance of some terrestrial and marine species, increased risk of heat-related human health impacts, and loss of infrastructure in the coastal regions and the Arctic.

STABILIZATION REQUIRES DEEP EMISSIONS REDUCTIONS

The report demonstrates that stabilizing atmospheric carbon dioxide concentrations will require deep reductions in the amount of carbon dioxide emitted. Because human carbon dioxide emissions exceed removal rates through natural carbon “sinks,” keeping emission rates the same will not lead to stabilization of carbon dioxide. Emissions reductions larger than about 80 percent, relative to whatever peak global emissions rate may be reached, are required to approximately stabilize carbon concentrations for a century or so at any chosen target level.

But stabilizing atmospheric concentrations does not mean that temperatures will stabilize immediately. Because of time-lags inherent in the Earth’s climate, warming that occurs in response to a given increase in the concentration of carbon dioxide (“transient climate change”) reflects only about half the eventual total warming (“equilibrium climate change”) that would occur for stabilization at the same concentration. For example, if concentrations reached 550 ppmv, transient warming would be about 1.6°C, but holding concentrations at 550 ppmv would mean that warming would continue over the next several centuries, reaching a best estimate of an equilibrium warming of about 3°C. Estimates of warming are based on models that incorporate ‘climate sensitivities’—the amount of warming expected at different atmospheric concentrations of carbon dioxide. Because there are many factors that shape climate, uncertainty in the climate sensitivity is large; the possibility of greater warming, implying additional risk, cannot be ruled out, and smaller warmings are also possible. In the example given above, choosing a concentration target of 550 ppmv could produce a likely global warming at equilibrium as low as 2.1°C, but warming could be as high as 4.3°C, increasing the severity of impacts. Thus, choices about stabilization targets will depend upon value judgments regarding the degree of acceptable risk.

CONCLUSION

This report provides a scientific evaluation of the implications of various climate stabilization targets. The report concludes that certain levels of warming associated with carbon dioxide emissions could lock the Earth and many future generations of humans into very large impacts; similarly, some targets could avoid such changes. It makes clear the importance of 21st century choices regarding long-term climate stabilization.

Record High Temperatures

One swallow does not a summer make, nor does a hot day mean that global warming is underway… but since climate change deniers in the US made a big deal of the snowstorms this winter, despite the fact that global warming should increase the chance of such storms, I can’t resist pointing out this item from the blog of meteorologist Jeff Masters:

June 2010 features an unprecedented heat wave in Asia and North Africa

A withering heat wave of unprecedented intensity brought the hottest temperatures in recorded history to six nations in Asia and Africa, plus the Asian portion of Russia, in June 2010. At least two other Middle East nations came within a degree of their hottest temperatures ever in June.

The heat was the most intense in Kuwait, which recorded its hottest temperature in history on June 15 in Abdaly, according to the Kuwait Met office. The mercury hit 52.6°C (126.7°F). Kuwait’s previous all-time hottest temperature was 51.9°C (125.4°F), on July 27,2007, at Abdaly. Temperatures reached 51°C (123.8°F) in the capital of Kuwait City on June 15, 2010.

Iraq had its hottest day in history on June 14, 2010, when the mercury hit 52.0°C (125.6°F) in Basra. Iraq’s previous record was 51.7°C (125.1°F) set August 8, 1937, in Ash Shu’aybah.

Saudi Arabia had its hottest temperature ever on June 22, 2010, with a reading of 52.0°C (125.6°F) in Jeddah, the second largest city in Saudi Arabia. The previous record was 51.7°C (125.1°F), at Abqaiq, date unknown. The record heat was accompanied by a sandstorm, which caused eight power plants to go offline, resulting in blackouts to several Saudi cities.

In Africa, Chad had its hottest day in history on June 22, 2010, when the temperature reached 47.6°C (117.7°F) at Faya. The previous record was 47.4°C (117.3°F) at Faya on June 3 and June 9, 1961.

Niger tied its record for hottest day in history on June 22, 2010, when the temperature reached 47.1°C (116.8°F) at Bilma. That record stood for just one day, as Bilma broke the record again on June 23, when the mercury topped out at 48.2°C (118.8°F). The previous record was 47.1°C on May 24, 1998, also at Bilma.

Sudan recorded its hottest temperature in its history on June 25 when the mercury rose to 49.6°C (121.3°F) at Dongola. The previous record was 49.5°C (121.1°F) set in July 1987 in Aba Hamed.

The Asian portion of Russia recorded its highest temperature in history on June 25, when the mercury hit 42.3°C (108.1°F) at Belogorsk, near the Amur River border with China. The previous record was 41.7°C (107.1°F) at nearby Aksha on July 21, 2004. (The record for European Russia is 43.8°C–110.8°F–set on August 6, 1940, at Alexandrov Gaj near the border with Kazakhstan.

Two other countries came within a degree of their all time hottest temperature on record during the heat wave. Bahrain had its hottest June temperature ever, 46.9°C, on June 20, missing the all-time record of 47.5°C (117.5°F), set July 14, 2000. Temperatures in Quatar reached 48.8°C (119.8°F) on June 20. Quatar’s all-time record hottest temperature was 49.6°C (121.3°F) set on July 9, 2000. All of these records are unofficial, and will need to be verified by the World Meteorological Organization (WMO.) The source for the previous all-time records listed here is the book Extreme Weather by Chris Burt. According to Mr. Burt, the only other time as many as six nations set their all-time highest temperature marks in a single month was during the European heat wave of August 2003.

Perhaps more important than these scattered jaw-dropping hot spots are the following facts from the US National Climatic Data Center:

The world land surface temperature June 2010 anomaly of 1.07°C (1.93°F) was the warmest on record, surpassing the previous June record set in 2005 by 0.12°C (0.22°F). The anomalous warm conditions that affected large portions of each inhabited continent also contributed to the warmest June worldwide land and ocean surface temperature since records began in 1880. The previous June record was set in 2005. Separately, the worldwide ocean surface temperatures during June 2010 were 0.54°C (0.97°F) above the 20th century average—the fourth warmest June on record.

In fact, the whole year has been hot…

But even more important are the long-term trends…

Of course, you need to read the paper to understand how this graph was made.

News About the Younger Dryas

I don’t want to write anything really interesting here until the technology gets upgraded…

… but I figure I might as well start puttle little comments about ecological issues here, instead of on my diary.

So:

• Chris Colose, Revisiting the Younger Dryas, RealClimate, July 17, 2010.

The Younger Dryas was, among other things, a sudden cooling event in Europe shortly after the end of the last ice age. In only 20 years, the temperature in Europe dropped about 7 Celsius! It stayed cold for about 1,300 years. In Greenland, the temperature went down 15 Celsius. And then, at the end of the Younger Dryas, temperatures in Europe bounced back just as fast.

Sudden climate changes of this magnitude could have a huge impact on human civilization – just imagine glaciers in the Lake District in England. So, it’s worth learning all we can about this episode. Indeed, some people have suggested that freshwater from melting ice was what brought on the Younger Dryas, by disrupting ocean circulation in the northern Atlantic… which raises the specter of a repeat of this incident sometime soon! Luckily, the chances of that now seem very low. But it’s still good to understand this stuff.

If you haven’t learned a bit about Heinrich events (when icebergs drop lots of rocks on to the floor of the northern Altantic), the Bølling-Allerød warm period that came right before the Younger Dryas, the Last Glacial Maximum or LGM around 20,000 year ago, and the Atlantic meridional overturning circulation or AMOC, Chris Colose’s comments may seem a bit dry and jargonesque. But I find them fascinating!

For one thing, I hadn’t known that people were finding evidence of Younger-Dryas-like episodes at the end of earlier glacial periods, suggesting that these events are in some sense routine, rather than something that requires a freak event like a comet impact to explain. (Yes, some people have argued that a comet was to blame.)