Ben Haller: Global Climate and the Amazon
This talk has accompanying images. Surf to http://tinyurl.com/4yv4bh and you will be redirected to my Mac.com home page. Double-click on the "amazon" folder and you will see seven JPEG files. Download them using the downward-pointing arrows on the right. They will be explained below. This talk is longer since it's my talk, so there. :->
Major regions such as the Amazon Basin have a large impact upon global climate, and they are greatly affected by global climate patterns; the interaction is bidirectional. These interactions have become of particular importance with the advent of global warming; understanding how climate change will affect the Amazon, and vice versa, is essential to minimizing the impact of global warming and planning for the future. Global warming is an enormous threat to the planet and to human civilization; for more on that, I refer you to Al Gore's recent movie, and to NewScientist's excellent coverage of the issue. In this talk, I will discuss the mechanisms by which global warming occurs, its projected effects upon the Amazon basin, and the Amazon's effects on global climate change.
The way that global warming works is discussed at great length on many websites; no doubt Wikipedia has thorough coverage of it. So I'll just touch on the basics here. Most of the energy from the sun is in the ultraviolet and visible wavelengths of light; other wavelengths are relatively minor contributors. A good deal of ultraviolet gets absorbed by ozone high in Earth's atmosphere; most of the rest of the light makes it to the lower atmosphere and to the surface of the earth, since air is very nearly transparent to visible light. Some (30% is the figure I recall from memory) is reflected directly back into space by clouds, by reflection off the oceans, and by the reflectivity (or "albedo") of land masses. The remainder is absorbed, acting to heat up the oceans and the land. Warm objects emit light, too, however, as blackbody radiation; at the temperature of the Earth, the emitted light is in the infrared. Air is not transparent to infrared radiation; in particular, the greenhouse gases, such as carbon dioxide, methane, and various others absorb infrared light and re-emit it in a random direction. This means that very little of that infrared light makes it back out into space; it is effectively trapped. The more greenhouse gases in the atmosphere, the more heat is trapped.
That's the simple picture. There are a few wrinkles. Water vapor is actually the most important greenhouse gas, but it is rarely discussed because it is not under direct human control; if there's too much water vapor, it rains, if there's too little, it water evaporates from the oceans, so the atmospheric water vapor content is governed by those sorts of factors (which is an oversimplification, like everything in this talk, but there it is). So carbon dioxide, mathane, and various oxides of sulphur and nitrogen -- the "anthropogenic," or human-generated, gases -- are the ones generally focused upon. Another wrinkle is that soot has recently been found to be a major contributor to global warming, contrary to previous thinking; a short piece by Service in Science (2008) discusses this. Aerosols are also quite important, but their effect is poorly understood at present.
The effects of global warming are similarly unclear. The big picture is easy: melting ice, rising sea levels, more extreme weather, water supply problems, and higher average temperatures over the globe. But the small picture, the effects of global warming on local microclimates, is still hard to predict. Most of the results discussed in this talk came out in only the last year or three, and they are preliminary and tentative. In general, however, it can be said that global warming will cause changes in atmospheric and oceanic circulation that will drive many local changes in climate (meaning rainfall, temperature, and seasonality).
What will the effects be on the Amazon specifically? Baettig et al. (2006) looked at the local effects of climate change worldwide, and the three images online from their paper show those effects. The first image essentially shows that everywhere will get hotter; if one takes the hottest year in a baseline period from the 1950s to the 1970s (if I recall correctly), pretty much the whole planet will experience 14-19 out of 20 years as being hotter than that baseline hottest year. What used to be an unusual, once in twenty years event will become the norm. The second image shows shifts in precipitation, and you can see from this that the Amazon will become much drier; many years in any 20-year period will be drier than the driest year in the baseline period. This is an extremely large shift in the climate of the Amazon. (Unfortunately, the same image shows that North America will largely be spared from major effects, so our politicians will be able to continue to ignore the issue.) The third image combines various factors into an overall "climate change index," which reveals that the Amazon will be affected by changing climate more than almost any other area except perhaps the Arctic and southern Africa.
What is driving this warming and drying of the Amazon specifically? I want to mention four factors. One is a shift from pastureland to soybean agriculture (Costa et al., 2007). This is being driven by the U.S. shift towards corn ethanol for biofuel production; land in the U.S. is being converted from soybeans to corn, raising the price of soybeans on the global market and causing Brazilian landowners to shift from cattle grazing to soybean farming. But soybean fields have a greater albedo than pasture; so more incoming sunlight is reflected directly back out into space, which (perhaps unexpectedly) causes less moisture to be retained by the land (I'm not sure how that works, exactly, sorry). This shows how interconnected global forces are, and how policy in the U.S. drives change worldwide in unexpected ways. The second factor is decreasing reflective aerosol production (Cox et al., 2008). For a long time now, dirty industrial activity such as the burning of coal has been emitting a lot of reflective aerosols into the atmosphere. This has had an overall cooling effect worldwide. But unfortunately, as industry cleans up its act and plants and factories become less polluting, the effect of lowered aerosol production will be an accelerated pace of warming. The third factor is deforestation (Malhi et al., 2008). Forest retains moisture; vegetation has a high moisture content, and it recycles precipitation back into the atmosphere. As deforestation progresses, more and more rainfall will go into rivers, and then to the ocean, or will go into underground aquifers; it will no longer be put directly back into the atmosphere by transpiration. The extremely humid environment generated by the trees of Amazonia will be lost. Deforestation along roads in the interior of the Amazon is particularly damaging in this respect, because it fragments the humid zones and drives further deforestation due to insufficient humidity in the interior. The last factor driving warming and drying is a shift in rainfall to a a shift in the position of the Hadley cell circulation in the atmosphere. In essence, incoming sunlight is strongest in the tropics, and this warms the air and causes it to rise. As it rises, it cools, and rain occurs due to the decrease in the amount of water the cooler air can hold. This is why the tropics get so much rain. The cooler, dry air falls back towards the surface at around 30 degrees north and south latitude; this is the reason for the deserts that occur at those latitudes, from the Sahara to the Sonora. But as global warming increases, the northern hemisphere is expected to get warm more quickly than the southern hemisphere, because it has more continental land area; the oceans act to stabilize temperatures and delay temperature changes. This means that global warming will cause the "tropical covergence" where the heavy rainfall occurs to shift northwards. The end result is that the rainfall mostly misses the Amazon; the Amazon will be in the zone that is now the desert zone. The first image from Malhi online shows probabilities of various amounts of rainfall reduction for the Amazon basin at two times of year. In December through February, the dry season in the northern Amazon, the image shows that rainfall is likely to decrease significantly in the northern region, while in June through August, the dry season in the southern Amazon, the image shows that rainfall is likely to decrease in the southern region. In other words, the rainfall will decrease the most during the dry season in each area, when it was needed the most; so the reduction in rainfall will be extremely harmful to the native flora. The second image from Malhi superimposes potential forest loss by 2050 on top of a map of drought probabilities under two different IPCC-defined future climate projections; it shows that heavy deforestation will likely occur in the southeast Amazon, where drought is also most severe. This means that the forest in that area will be hit doubly hard, and will dry out more quickly than the forest in the northwest Amazon.
What will this projected warming and drying of the Amazon do to its flora and fauna? The answer is not good; the expectation is that the forest will disappear, even if humans don't cut it all down, and the area will shift towards a sort of tropical savannah. Thomas et al. (2004) discuss the extinction risk faced by species due to climate change. Worldwide they predict that by 2050 "15-37% of species in [their] sample of regions and taxa will be 'committed to extinction'". The picture for the Amazon is even more dire; of plant taxa in the Amazon, they predict (this time under a maximum climate change scenario, unlike their global extinction prediction, which was based on a midrange scenario) that 69-87% will be committed to extinction by 2050. Their survey of Amazonian extinctions is based on a small number of species and is probably quite imprecise, but it is quite a scary prediction. Williams et al. (2007) compare worldwide climates today to those projected for 2100 and analyze what kinds of changes will occur. If a climate (a particular combination of temperature and rainfall profiles and seasonality) exists somewhere today but will not exist anywhere in 2100, it is called a "disappearing" climate, and species adapted to that climate will go extinct. If a climate will exist somewhere in 2100 but exists nowhere on earth today, it is called a "novel" climate, and will represent an opportunity of sorts for colonization and adaptation (probably by aggressive invasive species). The first image from Williams shows that the Amazonian lowlands will experience a novel climate in 2100: drying and warming will create a new type of savannah-like climate currently unknown in the world. The image also shows that the climate of the highlands near the Andes will disappear: the clouds that define the highland cloud forest will rise in altitude due to global warming until they have lifted right off the tops of the mountains, and the cloud forest will cease to exist, being replaced by a more typical montane climate regime. But this first image fails to capture an important biological aspect: dispersal. If a region changes climate, it doesn't really matter if that climate already exists halfway around the world; the species that are adapted to the new climate probably won't be able to migrate to the area from their refuge thousands of miles away. Species dispersal into new areas is limited by time and distance, and so when talking about novel and disappearing climates, it is really necessary to see whether a climate is novel or disappearing within a localized area in which dispersal of species could realistically occur. This is what the second image from Williams shows, and the picture is quite dire. From the perspective of species trying to migrate, disperse or adapt to climate change, the climate of the Amazon as it is today will disappear almost completely, both in the lowlands and the highlands, and will be replaced by climates that are completely novel for the flora and fauna anywhere nearby. The projection, then is massive extinctions of local flora and fauna, and wholesale conversion of the area to a low-diversity invasive. The local species will simply not be adapted to the climate in which they find themselves.
That, then, is the dismal picture of what the future holds for the Amazon. The other topic I want to briefly discuss is how the Amazon itself affects global climate. As the Amazon changes due to global warming, the changes it undergoes will have further repercussions that it is important to understand.
Malhi et al. (2008) discuss some of these effects; their summary is that the Amazon's "removal by deforestation can itself be a driver of climate change and a positive feedback on externally forced climate change." The rainforest stores a great deal of carbon by capturing carbon dioxide from the atmosphere and converting it into biomass (wood and leaves), a good deal of which ends up semi-permanently stored in the soil. Deforestation both halts the deposition of biomass into the soil layer, and releases all of the carbon that was stored in the trees (assuming the trees are burned, as is typically the case with the slash-and-burn agriculture responsible for much of the Amazonian deforestation). Deforestation will therefore enhance anthropogenic increases in atmospheric carbon dioxide. Rainforest also extracts soil water and returns it to the atmosphere by transpiration; "large-scale forest loss tends to reduce rainfall" as a result, contributing to regional drying as mentioned earlier. Loss of rainforest, according to Malhi et al., will also reduce cloud cover, increase land surface albedo, increase atmospheric aerosol content, and change wind speeds and patterns. The implications of many of these effects for global climate are hotly debated.
A final consideration is the Amazon's effect on the thermohaline circulation (THC). The THC is a global system of oceanic flow that conveys heat from the tropics into the Arctic and Antarctic regions; the Gulf Stream is the best-known component of the THC for most people in North America, but there are many other similar streams, both on the surface of the ocean and at depth. The water that flows out of the Amazon basin in the Atlantic enters the same stream that becomes the Gulf Stream further north (I have no idea what it's called down here). There has been a great deal of speculation that as the Arctic and Greenland ice melts, that massive input of cold freshwater into the THC system may cause it to halt, because its flow is driven by temperature and salinity differences in different parts of the ocean. The effects of a halt (or even a slowing) of the THC would be quite dire, perhaps including glaciation of a good deal of western Europe, which is warmed by the waters of the Gulf Stream. According to Stouffer et al. (2006), however, the input of freshwater from the Amazon has the same effect, even though the input occurs so much farther south. As the Amazon dries and rainfall in the region decreases drastically, the freshwater input from the Amazon basin will decrease accordingly; this decrease may offset the increase in freshwater input in the Arctic region, and prevent a slowdown or stoppage of the THC. Stouffer et al. don't explicitly point out this relationship; I am conjecturing based upon their data, but it seems to me that this is a (single, solitary) positive consequence of the likely disappearance of the Amazon as we know it. It's as silver of a lining as we're likely to see on this topic, so I'll end on that.
People often speak of a tipping point in global climate, a level of atmospheric carbon dioxide, or an amount of temperature increase, below which things will be more or less OK, and above which feedbacks and interactions will cause the Earth's climate to change in profound and irreversible ways. In researching this topic, however, global came to look, to me, more like a series of dominos. The Amazon may be one of the first dominos to fall, since it is being pushed not only by climate change but also by deforestation, and by positive feedbacks inherent to its climate; as the studies I've presented show, there is every reason to believe it will be disappearing fast by 2050 and essentially gone by 2100. The Arctic sea ice is another domino likely to fall soon; some estimates predict that the Arctic may be open ocean i summer as early as 2030, although most predictions fall somewhat later. Both of these dominos will cause further dominos to fall. As Malhi et al. point out, the drying of Amazonia "could greatly expand the area suitable for soy, cattle, and sugarcane, accelerating forest disappearance" and generating more methane, fertilizer runoff, and other drivers of environmental degradation. As the Arctic converts to open ocean, countries are already competing for the rights to drill the vast oil reserves that have long been inaccessible under the Arctic ice, and burning all of that oil will, of course, drive further global warming. In my opinion, then, we should not think in terms of a tipping point, but in terms of dominos, and it is of great importance to try to prevent the first dominos from falling. The Amazon depends upon us for its survival; but conversely, we may also depend upon it for ours.
Williams, J.W., Jackson, S.T., & Kutzbach, J.E. (2007). Projected distributions of novel and disappearing climates by 2100 AD. Proceedings of the National Academy of Sciences, 104(14), 5738-5742.
Thomas, C.D., Cameron, A., Green, R.E., Bakkenes, M., Beaumont, L.J., Collingham, Y.C., et al. (2004). Extinction risk from climate change. Nature, 427(6970), 145-148.
Stouffer, R.J., Yin, J., Gregory, J.M., Dixon, K.W., Spelman, M.J., Hurlin, W., et al. (2006). Investigating the causes of the response of the thermohaline circulation to past and future climate changes. Journal of Climate, 19(8), 1365-1387.
Service, R.F. (2008). Study fingers soot as a major player in global warming. Science, 319(5871), 1745.
Malhi, Y., Roberts, J.T., Betts, R.A., Killeen, T.J., Li, W., & Nobre, C.A. (2008). Climate change, deforestation, and the fate of the Amazon. Science, 319(5860), 169-172.
Cox, P.M., Harris, P.P., Huntingford, C., Betts, R.A., Collins, M., Jones, C.D., et al. (2008). Increasing risk of Amazonian drought due to decreasing aerosol pollution. Nature, 453(7192), 212-215.
Costa, M.H., Yanagi, S.N.M., Souza, P.J.O.P., Ribeiro, A., & Rocha, E.J.P. (2007). Climate change in Amazonia caused by soybean cropland expansion, as compared to caused by pastureland expansion. Geophysical Research Letters, 34, L07706.
Baettig, M.B., Wild, M., & Imboden, D.M. (2007). A climate change index: Where climate change may be most prominent in the 21st century. Geophysical Research Letters, 34, L01705.