Forestclimate Feedbacks In The Greenhouse World

It is generally agreed that the world will warm by several degrees Celsius over the next century or so, as a result of the extra greenhouse gases that humans are putting into the atmosphere. If forest cover stays just as it is now, it is likely to play a role in setting up this new climate through its own feedbacks. And if forest vegetation begins to spread in the greenhouse world, it may well further amplify changes just as it did during the Quaternary, by adding to the warming in high latitudes. For instance, a modeling study by Levis and colleagues suggested that the spread of forests over tundra in response to an initial greenhouse effect warming will eventually warm the spring climate in these northerly regions by an additional 1.1-1.6°C, on top of the 3.3°C warming expected from the greenhouse effect over the next few decades.

In some regions of the world, the influence of forests might already be altering the path of warming perceived in recent climate station records. Just measuring temperature is not necessarily a good way of assessing changes in the heat balance of a particular region. This is because a lot of heat energy is made "invisible" by getting taken up as latent heat during evapotranspiration from forest canopies. Even if more heat is being trapped by the greenhouse effect in a particular region, it might not show up because the forest simply evaporates more water, which takes up the heat and exports it to other parts of the world. If one looks at trends in total heat flux—both temperature and latent heat of evaporation—several regions of the USA are actually warming more quickly than would be expected from looking at temperature alone. By evaporating water that temporarily takes up heat, forests may be disguising the true extent of climate warming.

Recycling of rainwater by transpiration from vegetation across Europe may help to increase the amount of summer variability in both temperatures and rainfall over the region as global warming accelerates during the coming decades, according to a recent modeling study by Senevirante and colleagues. It seems that, with the strong positive feedbacks involving rainfall, evaporation and cloud cover, the summer climate will tend to flip between extreme states: either very hot and dry, or cool and very wet. Some have suggested the recent series of extreme summers in Europe shows that this pattern is beginning to manifest itself.

Another aspect not generally considered is that changes in land use over the coming decades might significantly alter the path of warming away from the trends forecasted by standard climate models. If deforestation in Amazonia continues until all its forest cover has been replaced by grassland, this will alter the time course of climate change in the region. While the greenhouse effect alone would tend to warm the climate in the Amazon Basin by about 2°C, the temperature increase from losing the forest (due to decreased latent heat uptake) would be around 1.4°C, according to a modeling study by Jonathan Foley and colleagues at the University of Wisconsin. This extra warming adds up to a considerable increase in temperature, in an already-warming tropical climate where higher temperatures tend to decrease the photosynthesis and growth of plants. It is quite possible that any remaining fragments of the rainforest ecosystem would be wiped out by this warming, and it would not be much good for the croplands either.

Conversely, cutting down the world's rainforests might actually help preserve the mid and higher latitudes against global warming. Since latent heat from the rainforests helps keep the higher latitudes warm, cutting off this heat source might help to counteract some of the warming from the greenhouse effect. However, this would be of no benefit to tropical countries which would have to sweat out the climatic consequences of losing their forest cover, and we must also consider all the extra carbon from the destroyed forests that would enter the atmosphere as CO2, adding to the warming everywhere in the world. All things considered, the mid-latitudes might actually end up warmer rather than cooler overall if the tropical forests were cleared.

On the other hand, what if world forest cover becomes more widespread in the future? This will in itself tend to cause a warming of global climate irrespective of any greenhouse effect warming, because forest has a lower albedo than cleared land. If all the world's deforested land were allowed to return to its natural forest cover, it might warm the world overall by 1.3°C due to its effects on albedo, according to a recent modeling study by Gibbard and colleagues. Because tropical forests soak up so much heat in transpiration, this change would barely be felt in the tropics and the warming would mostly occur at higher latitudes (part of the warming there would be a result of latent heat reaching them from the increased tropical forests). On a global scale 1.3°C

is a large warming—not very much less than some of the forecasts for the greenhouse effect over the next century, and larger than the warming that has occurred since the mid-1800s that has caused so many changes around the world. In addition, as well as transporting more latent heat around, more forest will mean more water vapor in the atmosphere at any one time, and water vapor is a potent greenhouse gas that might further add to the warming. This scenario of increased water vapor in the atmosphere from more forest cover is another factor that needs to be fully considered in climate modeling, if we are to carefully weigh all the options. There is at present a lot of interest in planting forests to soak up carbon dioxide, to prevent some of the warming from the greenhouse effect (see Chapter 7). Yet, as Roger Pielke and his co-authors have pointed out, with all the vegetation-climate feedbacks this increased forest cover might actually warm the planet by more than the CO2 that it soaks up!

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