In the Abstract to the paper Massive Forest Dieback (pdf) ecologist Craig Allen of the U.S. Geological Survey points out that "most field studies and model-based assessments of vegetation responses to climate have focused on changes associated with natality and growth, which are inherently slow processes for woody plants—even though the most rapid changes in vegetation are caused by mortality rather than natality." A previous page on this subject, North American drought worst in 500 years includes a survey map which shows the results of a partial survey of the Rocky Mountain Ecosystem and indicates ongoing and widespread tree mortaility taking place in America's forests. Massive forest diebacks are taking place all over the American West, and it would seem that this taking place with little public awareness of the scale of the problem, since you don't hear much about the developing situation outside the affected areas. It was this developing die-off the ecosystem, and the state of America's forests that prompted the sudden interest of the Bush Administration in trees, and resulted in what was called the (Un)Healthy Forests Initiative, one of those Orwellian sounding government programs (Healthy Forests?) which represents both a response to the problem as well as an attempt to hide both the scale and the true nature of the problem (the use of the term 'Healthy Forests'). The solution proposed (increased logging) and the supposed root cause of the problem as described (bad environmentalists who prevented proper management of the forests through logging) failed to address the real root cause of the difficulties being experienced in the forests. The dieback is being caused by a severe climate shift. In the debate over global warming, there is much talk about traumatic and extremely damaging 'transient' shifts in climate on the road to a 'new equilibrium' and so the real question about the shift in climate in the American west as it relates to the global warming issue is whether or not this could be considered the new climate of the American West (a severe desert) or one of those rapid and sudden transient climate regimes that strike unexpectedly on the road to some new, and unknown climate regime. (See the page Our Changing Climate : a summary review for examples of other rapid and sudden climate changes that have taken place concurrent with this dieback in the American West ... this page includes an interesting comment received from a climatologist who seems to support the traditional equilibrium model of climate change...one must recall that rapid climate change is outside the mainstream as far as climatology goes today (see the references in the comments to scientific executive summary of 2001 where the authors state that due to the lack of research they can only warn about the possibility of these sudden dramatic shifts in climate, but they can provide no detailed analysis since there were no papers on the subject available for summarization).

The question then becomes is this one of those harmful and sudden transient climate shifts or is this the new climate equilibrium for the American West, and no scientist in the world would have the answer to that question, but either way, the end result is extremely damaging and given how even a transient shift in the climate can then create new conditions that create a type of complex feedback loop, the transient winds up influencing the end result (the eventual equilibrium of the new climate regime, here assuming given the constant addition of further stimulus which increases the green house effect that there would even be a chance to achieve equilibrium - here we can see that if the dieback is a transient, the feedback comes in the form of destruction of the hydrological cycle, in that the term 'rain forest' is meaningful for a good reason, in that vegetation exhales water into the atmosphere in order to keep a siphoning action going which works from root to leaf, and thus the transient then influences the final equilibrium in that there is now certain to be even less rain due to the transient, if that is what is was, which destroyed the rain maker, the forests. Simple common sense would then indicate that the final result is therefore more likely to be equilibrium (assuming that this climate can ever achieve equilibrium given the increasing demand for fossil fuels) in the form of a low rainfall environment (the only rain produced then dependant on the oceans, since the forests were dead, and could no longer produce rain).

As for the 'Healthy Forest Initiative', it is unfair to blame the problem on 'environmentalists' and 'lack of logging', when actually it was environmentalists who were arguing that the forest fire suppression activities intended to benefit the logging industry (by saving 'valuable timber from fire') was creating dangerous fire conditions, and fires should be allowed to burn naturally. To ignore the industrial contribution to fire conditions and then blame enviromentalists is just partisan politics and one of those schemes that come out of the White House (was this one thought up by Karl Rove?) which attempts to obtain a political advantage and assign blame rather than objectively dealing with the real problems in America's forest policies. Furthermore the real problem is drought, and further deforestation, on top of the already serious dieback which is underway, is no way to minimize a drought. As well the suicidal policy of cutting in the last remaining stands of old growth rain forests should be halted. While even one old growth tree is worth close to a quarter of a million dollars, they do not call these 'old growth rain forests' for no good reason. These ancient trees move the most water into the atmosphere and the power of their rain making ability is evident to anyone who has experienced a really good soaking in the forest (or tired to keep their gear dry, which is almost impossible). Deforestation of this type which is taking place around the world is also an element in the feedback loop, which then contributes further to the end result of climate change (reduced precipitation, forests are part of the carbon cycle, as well as the lungs of the planet, increased susceptibility to natural disasters such as flooding and mudslides, and so forth).

The dieback in America's forests is the most severe in Southern California, and given the developing conditions, it looks to be the case that Southern California will lose all its forests. The situation is also very bad and getting worse in the Rocky Mountain ecosystem, as a holocaust of sorts is destroying millions and millions of trees, which once again becomes an element in a feedback loop, both in that these dead trees feed wild fires, and there is then also less rain due to the loss of rain making trees which then feeds back to kill even more trees.

This feedback loop then results in an exponentially increasing rate of dieback as the effects compound, as discussed in the Allen paper linked to above. Speaking of this sort of 'nonlinear' feedback loop and thresholds, which once exceeded cause rapid changes in an ecosystem, he wrote, "Drought-induced tree mortality exhibits a variety of nonlinear ecological dynamics. Tree mortality occurs when drought conditions cause threshold levels of plant water stress to be exceeded, which can result in tree death by loss of within-stem hydraulic conductivity (Allen and Breshears – in press). Also, herbivorous insect populations can rapidly build up to outbreak levels in response to increased food availability from drought-weakened host trees, such as the various bark beetle species (e.g. Dendroctonus, Ips, and Scolytus spp.) that attack forest trees (Furniss and Carolin 1977). As bark beetle populations build up they become increasingly successful in killing drought-weakened trees through mass attacks (Figure 1), with positive feedbacks for further explosive growth in beetle numbers which can result in nonlinear ecological interactions and complex spatial dynamics (cf. Logan and Powell 2001, Bjornstad et al. 2002). Bark beetles also selectively kill larger and low-vigor trees, truncating the size and age distributions of host species (Swetnam and Betancourt 1998)."

It is interesting to note here that the dieback of America's forests is now being discussed in mainstream media (for example there was story in Time magazine in August) and the theme of these stories is that 'drought happens all the time' and not once is the forbidden phrase 'global warming' or 'climate change' ever mentioned, even as a possible explanation. It is worth noting here that the debunking of 'global warming' is now obsolete. In order to debunk 'global warming' it would be required that one insist that one can increase green house gases without increasing the greenhouse effect. These Greenhouse gases are the tiniest component of the planet's atmosphere, a very small percentage of the gases in the atmosphere, and their potency in even these small concentrations is evidence for the power of the greenhouse effect. (These same gases are present in high concentrations in the atmosphere of Venus, with the result being that the surface temperature of Venus is close to 800 degrees.) Fortunately on Earth the percentage of these gases is tiny, thus making the planet livable, and when these gases are increased by between one third and one half (and projected to double in the future) one can of course expect an increased green house effect. For these reason the debunking of 'global warming' requires the debunking of 'the greenhouse effect', and this would be psuedo-science. The proper course of action then is to suggest, as the mainstream press reports as one example, that 'climate change is natural and drought happens all the time'. A common debunking argument suggests that while human beings have increased green house gases by a large amount, nevertheless we are in an 'interglacial' and therefore most of the warming taking place is natural, and the human component is quite small. This, by the way, seems to be the new archetype for debunking global warming, combined with the argument that 'climate change is natural'. This contradicts the evidence for 'equilibrium' and it would seem that the preference for gradual climate shifts and equilibrium is prevalent in climatology studies for the simple reason that for thousands of years the climate has been in a state of equilibrium with very little forcing of the climate. Studies of ice core gases have revealed that during an ice age the amount of green house gases in the atmosphere is lowered and during the interglacial period the amount increases, with a type of recurring feedback loop taking place over a time frame typically covering thousands of years for the transition. Thus nature does not do much 'forcing' of the climate, and given how small the amounts of greenhouse gases are in the atmosphere, such a tiny percentage, it does not take much change one way or the other to switch between glaciation and the interglacial period. So then to debunk the human contribution to global warming requires one to debunk the effects of rapid forcing which is what is taking place (if nature was to pump that much CO2 into the atmosphere, studies of the past indicate that a climate shift would take place, and so then how can one argue that such rapid forcing as we see taking place due to human activities would not result in climate changes. This newest version of the debunking argument does not make sense on a number of levels, and is just a form of denial as far as I can see, and thus it is not helpful but just another one of those stalling tactics that does not help the situation whatsoever).

One of the big controversies in climatology surrounds the issue of 'rapid climate change' versus the 'equilibrium model', and there is also the poorly understood question of 'transients' (sudden rapid changes in climate that take place when thresholds are exceeded, and which then in a complex pattern result in feedback loops that create unexpected outcomes, or 'surprises' in the field of climatology). According to what was written in the Climate Change 2001: Working Group II: under the section Impacts, Adaptation and Vulnerability rapid climate change is currently outside the mainstream of climatology, and the end result, as far as I can see, is the propogation of a type of 'equilibrium' model which gives the public the impression that in gradual manner, over say fifty or one hundred years, the climate will change. This leaves the public unprepared for the possibility of sudden, rapid climate change when thesholds are exceeded, as well as vulnerable to the shocks of sudden transients, and then there is the issue of those feedback loops and sudden surprises. The working group produced an 'executive summary' of a host of papers written in the field of climatology, which was intended for policy makers and interested readers who would not be likely to read through piles of scientific papers. They note that they were unable to provide anything more than vague warnings about the possibility of sudden, rapid climate changes due to what they called 'the paucity of literature' due to a lack of research on this subject (rapid climate change is currently outside the mainstream of climatology at this time, for it is was mainstream, then there would be papers to summarize). Commenting on this conservatism within climate science, they wrote, ""Because of the magnitude of their potential consequences, large-scale discontinuous responses warrant careful consideration in evaluations of climate change dangers. Working Group II points to the potential for such occurrences and their potential consequences for human and natural systems, but it is unable to provide detailed assessments of potential effects, given the paucity of information in the literature." They also state that, "Transient scenarios are just entering the climate impacts literature, which unfortunately tends to lag the climate effects literature by several years; thus, much of the impacts literature still is based on equilibrium climate change scenarios." And here we get those climate simulations that model gradual shifts in the climate that take 50 to 100 years to take place. In the American West we can see rapid changes taking place, and the establishment of feedback loops leading to 'nonlinear' rates of change, and the same pattern has emerged in ecosystems around the world, all at the same time (the late 1990s, as I attempted to point out in the summary page linked to above). Now if you point this out you are condemned for being 'alarmist' (exactly the criticism the climatologist made in his comments on the summary page I posted on rapid concurrent climate changes, linked to above).

However there is all sorts of evidence to indicate that the equilibrium model is incorrect, and given the forcing taking place (rapid increases in CO2) the possibility of transients and rapid or sudden climate changes increases. As well there is good evidence that the climate is a system with thesholds. IN such a system there would be small changes, as the system attempts to maintain equilibrium, and then suddenly changes take place once the threshold is crossed. Various feedback loops then feed into the process leading to 'strongly nonlinear responses' which are not slow and gradual, but rather these feedback loops can actually accelerate the process (as described above...another example of the same sort of process is the melting of glaciers or ice caps...melt water absorbs heat while ice reflects heat, so that as melting occurs, a feedback loop then begins which accelerates melting, and so we see that at the end of the 1990s south American glaciers began to melt at a rate 32 times faster than in the previous three decades, and the melt in the artic experienced a similar acceleration (about 20 times faster). It is ironic that the dumping of this much cold melt water into the ocean could then shut down the Atlantic conveyer belt currents, which brings tropical water to Europe, through a type of sinking action in the North Atlantic (as the water cools, it sinks, pulling more water up from the tropics), leading to one of those transients, in the form of a mini-ice age, a temporary effect of global warming leading to very bad winters in Europe.




Global warming during the Industrial Age


Above is the famous global warming graph, which shows how temperature has risen during the age of industrialization, and human forcing of the climate regime through rapid increases in greenhouse gases. The graph remains a source of controvery, and can be interpreted in a number of ways. You will notice, that if you ignore the 'background noise' (the sharp up and down spikes in the graph) the climate is revealed to be one of those 'choatic systems' where order emerges (one of the interesting aspects of what is known as chaos theory, where somehow a system, like the weather, which is chaotic, produces order, in this case the order being the climate regime within which chaotic weather exists). Here you can see that during the 1800s the graph is pretty much flatlined. Then at around 1900 the graph begins to climb. Once again the graph flatlines in the 1940s, and then around 1970 it begins once to climb. Thus we can see both an equilibrium reponse in the climate (the flat line areas of the graph) and a strongly nonlinear response in the climate (the rapid rises seen in the graph).

The reason for this response remains a source of controversy, but one theory that does fit the evidence is as follows. If the climate is a system with thresholds, and which attempts to maintain equilibrium, even under conditions of forcing, then one would expect the climate to not change much for a period of time. When a threshold was crossed one would expect one of those strongly nonlinear responses which are characteristic of systems with thresholds. A system with thesholds shows almost no response before the threshold is crossed, and then suddenly all that pentup energy is released in the form of a strongly nonlinear change.

Other explantions for the strangeness of the global warming graph have been proposed. However the data does fit the rapid climate change scenario, and if this was true that means that what we are experiencing around the world in recent years is part of the process of global climate change, and given the particularly rapid changes taking place in ecosystems around the world since the late 1990s, it would then follow that yet another threshold has been crossed, which would then relate the American forest dieback, which began at the end of the 1990s, to the accelerated melting of the glaciers and ice caps, which began in the late 1990s, as well as many other rapid changes (the rapid retreat of the Sahara as one more example, which began at the same time). All these strongly nonlinear responses would then be seen as related to one another, and the common media explanation for America's forest problem would be revealed as bogus (there are always droughts, it is said, while not even once is 'global warming' or 'climate change' mentioned).

Given the importance of the oceans to climate (with even far away oceans influencing countries that don't even have a shoreline on those oceans) it would also seem that these strongly nonlinear responses are also related to the sudden rise in temperature of the tropical Atlantic over the last four years, shown in the graph below.




Rise in temperature of the Tropical Atlantic from 2000 to 2004


However, as I said, rapid climate change has not been in the mainstream of climatology, and in history, it has often been the case that what has been mainstream science offers resistance to what is once on the margins, only to move into the mainstream later, so the fact that 'alarmist' scenarios about 'rapid changes' and 'transients' and 'feedback loops' are set aside in favor of the currently popularly understood model of smooth changes over 50 or 100 years does not make that scenario true, and might lead to both inappropriate levels of public apathy as well as some really unpleasant surprises in the future, as people find themselves caught up in surprising feedback loops or enduring nasty transients, and rapid climate changes related to rapid forcing of a system which it turned out had thresholds, which while it might not have changed on them before, when it finally got around to releasing all those pent up changes it gave them some very unpleasant surprises, and if those feedback loops compound they might also find themselves enduring the difficult task of adapting to increasingly rapid climate changes which it turns out were only being stored up, even though it might have seemed at the time that not much was happening.

And we can see practical examples of how even the small changes in climate we have seen so far can have catastrophic effects, as the retreat of the Sahara over the last few years has now created a locust plague which in its infancy is already being called worst than the worst year of the previous locust plagues. The FAO is now talking about famine, which isn't surprising, especially when you consider the social factors that combine with climate change to create disasters (bad debts, bad policies, and a rapid climate change - any one of these would be bad enough, but together they create a disaster on a very short time scale making adaption difficult, and given the lack of a response to the growing crisis we can see then that in the time of climate change humanity will no doubt just be forced to endure one horrible disaster after antoher with no help forthcoming, if this locust plague is going to be the archetype of what is to come. This is worth considering since it is a common strategy of global warming debunkers to talk about the rosy scenario of long growing seasons, and to hail what they call 'human adaptability', but when push comes to shove the entire system proves to be lethally nonadaptive, and what you wind up with is 'Social Darwinism'. (See the page Plague Locusts poised to destroy Sahelian Harvest for a discussion of how bad policy combines with climate change to create a particularly bad disaster.)

Tree Mortality

The graph above is an illustration of of the effect of a climate transient, whereby as the climate moves into a new equilibrium a transient condition exists which kills the forests.


The pictures below show dead forests, along with graphs which show the nonlinear increase in tree mortality as drought conditions push the forests over the mortality threshold, with the large increase in dieoff beginning in 2003, which must have been the year the threshold began to be crossed in many forest areas. Once a threshold is crossed the entire forest area can die in as little as four months.




Dead Pinyon Pine Forest Forest




Dead Pinyon Pine after dropping needles




Pinyon Pine Mortality




Dead Lodge Pole Pine Forest




Lodge Pole Pine Mortality




Dead Ponderosa Pine Forest




Ponderosa Pine Mortality




Mt. Graham Telescope surrounded by dead forest




Map of Forest Dieback 1997-2003




Cumulative Graph of yearly dieback


The graphic above and the graph below show the forest dieback trends (pdf) in the United States, and the graph, which displays the yearly trend, shows a strongly nonlinear increase in the rate of forest dieback, which rose to 14 million acres in 2003. The North American continent has about 700 million forested acres, and if the trend continues, as well as the increasing rates of dieoff, this could result in significant damage occuring rapidly over a very short time span (this being (possibly) a climate transient which has a very damaging impact over a decade and not over a span of 50 or 100 years - the time span people usually use when referring to the impacts of climate change, this sort of time span being the result of the equilibrium modeling of climate impacts, and it assumes slow and gradual shifts, rather than rapid climate changes and damaging transients).





The damage from dieback is even more extensive in British Columbia (10 million acres in 2003, as compared to 14 million acres for the entire United States west) and this is an illustration of the potential effects of a nonlinear rate of increase in forest dieoff, a type of climate transient of which the impact occurs with rapid suddenness.


According to the USDA Forest Service report on Forest Health Climate Interactions (pdf) drought is the major influence in forest dieback events. According to the scientific literature, "depth of snow-pack was the only parameter of 10 regional climate stresses that showed a consistent link to forest dieback...snow depth was statistically significant for both the onset and recovery of dieback."





In the graph above you can see that as snow pack increases (the blue line) forest dieback decreases (the red line) and similarly as the snow pack drops the forest dieback peaks.





As far as global climate stresses go, forest dieback is shown to be strongly linked to the ENSO cycle (oscillations between El Nino and La Nina). Once again the reason for the dieback is reduced precipitation.




Drought Monitor


In the graphic above darker the color the more extreme the drought, with the blackish areas being 'esceptional' and the red areas being 'extreme' and the orange areas being 'severe'. Such severe conditions have persisted since the late 1990s. It goes without saying that trees and forests can only take so much of this type of extreme abuse, and are currently enduring the one of the worst droughts in recorded history. In order to see a forest dieback event that might be equal to what could be about to take place in North America it would be required to look back at least 13,000 years, and perhaps even tens of thousands of years.

IN terms of both regional and global climate influences, drought is the major influence over forest dieback. IN the case of the North American west the dieback is occuring over an area stretching from British Columbia in the North to the American South West, and drought is the common cause for the wide spread dieback in these various diverse ecosystems. The graphic above shows the extreme drought conditions being endured by the forest ecosystems of the West, a condition that has prevailed since the late 1990s, and it was inevitable that eventually forest ecosystems would be pushed over the mortality threshold, due to the year over year exposure to such extreme conditions. This long lasting drought is not related to the ENSO cycle. It is also worth noting that the ENSO cycle has also collapsed and now there is almost no cycle to speak of as La Nina has pretty much disappeared and now rather than the cycle which existed in the past there are only repeating El Ninos (so then the Southern Oscillation is no longer an oscillation).

Lack of water is the main cause of forest dieback, but not the only cause. Forest dieback in places like Haiti, parts of Africa, or India are caused by economic conditions, which lead to deforestation, which leads to drought, which then leads to desertification (for example in Haiti and parts of Africa, forests are clear cut over the years in order to make a little money selling charcoal). A forest dieback in Croatia is linked to "the anthropogenetic impacts presented by the pollution of air, water and soil, all causing stress and finally, death of forest trees; in addition, various engineering operations cause severe change in their habitats... the forest dieback is the result of industrial, urban, traffic and agricultural pollution, partly though, by the technologies badly adapted to forest ecosystems. The micro-habitat methods helped to assess great changes in the forest soils, dry and moist sulphate deposits, nitrates and other poisons being present in Croatian forests for a considerable time now. Though different tree species react differently, sooner or later all will be destroyed."

Insects and diseases are also causes of dieback, but are usually secondary effects, as these agents target drought weakened forests, often launching mass attacks, and drought weakened forests are also more susceptible to environmental toxins and engineering operations that damage the forest ecosystem.

The IPCC discusses scenarios for climate induced forest impacts

The impact of forest loss goes beyond just simply losing jobs or losing vacation spots, and the IPCC attempted to summarize a large number of papers in the field to prepare a summary of available scenarios which attempt to forecast the impact of climate change on the forests and then the impact of forest change on human society and larger ecosystem. It is worth nothing here that it would seem, if you read through the report, that many of its assumptions and conclusions seem to be both confirmed and outstripped by rapidly occuring events (for example, predictions are made of forest dieoff which occurs decades in the future, making the prediction of a dieoff accurate although it would appear the time scale was to conservative, since that has been taking place only a few years after the report was released in 2001). Many computer simulations predicted increasing and extreme rains in the American West, and it remains to be seen whether the destruction of the forests is a climate transient, and the eventual climate equilibrium is a wet climate, or if those climate simulations were themselves all wet (the report notes that in the field there are two competing scenarios, wetter or dryer).

According to the Intergovernmental Panel on Climate Change ...

"Forests hold about 62–78% of the world’s terrestrial biospheric carbon (Perruchoud and Fischlin, 1995), about 14–17% of which is in the forests of North America; about 86% of that is in the boreal forest (Apps et al., 1993; Heath et al., 1993; Sampson et al., 1993). Forests play a large role in global water and energy feedbacks (Bonan et al., 1995) and account for most of the world’s terrestrial evapotranspiration, which is about 64% of the precipitation (Peixoto and Oort, 1992; Neilson and Marks, 1994). Most of the world’s freshwater resources originate in forested regions, where water quality is directly related to forest health."

http://www.grida.no/climate/ipcc/regional/192.htm
Annual tree mortality losses from insect outbreaks in Canada are about 1.5 times the losses from wildfire and amount to about one-third of the annual harvest volume (Fleming and Volney, 1995). Annual losses from insects and fire in the United States also are about one-third of the annual harvest (Powell et al., 1993). Warming-induced changes in the timing of spring frosts may be important in ending or prolonging outbreaks. Increased drought stress also may enhance insect outbreaks, and changes in climate could extend the ranges of some insects and diseases.

http://www.grida.no/climate/ipcc/regional/193.htm
North American forests also are being subjected to numerous other stresses, including deposition of nitrogen and sulfur compounds and tropospheric ozone, primarily in eastern North America (Lovett, 1994). The interactions of these multiple stresses with elevated CO2 and climate change and with large pest infestations (of, for example, the balsam wooly adelgid, gypsy moth, spruce budworm, and others) are very difficult to predict; however, many efforts are under way to address these questions (Mattson and Haack, 1987; Loehle, 1988; Fajer et al., 1989; Taylor et al., 1994; Winner, 1994; Williams and Liebhold, 1995). Anthropogenic nitrogen fixation, for example, now far exceeds natural nitrogen fixation (Vitousek, 1994). Atmospheric nitrogen deposition has likely caused considerable accumulation of carbon in the biosphere since the last century (Vitousek, 1994; Townsend et al., 1996). However, nitrogen saturation in soils also can be deleterious, possibly causing forest dieback in some systems (Foster et al., 1997). Tropospheric ozone also can damage trees, causing improper stomatal function, root death, membrane leakage, and altered susceptibility to diseases (Manning and Tiedemann, 1995). Such ozone-induced changes can render trees more sensitive to warming-induced drought stress (McLaughlin and Downing, 1995). There are many other stress interactions, and researchers think that, in general, multiple stresses will act synergistically, accelerating change due to other stresses (Oppenheimer, 1989).

It is interesting to note the IPCC report was released in 2001 and the strongly nonlinear rate of forest dieback did not appear until 2003, which is something to keep in mind when considering the scenarios being discussed in the scientific papers being summarized in the report.

Writing of computer simulations of the climate they describe

http://www.grida.no/climate/ipcc/regional/196.htm
"two contrasting scenarios of the North American forest future must be considered: one with considerable forest dieback, another with much enhanced forest growth. These contrasting scenarios represent endpoints on a spectrum of possible responses...These results suggest the possibility that early forest responses to global warming could exhibit enhanced growth; later stages could produce widespread decline or dieback. Most combinations of scenarios and CO2 effects produce intermediate scenarios, with a regional mosaic of forest dieback and enhanced forest growth...Forests cannot move across the land surface as rapidly as the climate can. The faster the rate of climate change, the greater the probability of ecosystem disruption and species extinction. Were temperature-induced drought dieback to occur, it likely would begin shortly after observable warming; if accompanied by short-term precipitation deficits, it could occur very rapidly."

One the main human impacts of deforestation is the effect on the water supply.

http://www.grida.no/climate/ipcc/regional/200.htm
Important vulnerabilities of water resources to potential climate change scenarios involve changes in runoff and streamflow regimes, reductions in water quality associated with changes in runoff, and human demands for water supplies. Seasonal and annual runoff may change over large regions as a result of changes in precipitation or evapotranspiration.

http://www.grida.no/climate/ipcc/regional/200.htm
Runoff is simply the area-normalized difference between precipitation and evapotranspiration; as such, it is a function of watershed characteristics, the physical structure of the watershed, vegetation, and climate.

The world's forests are responsible for the majority of the evapotranspiration (plants exhaling water into the atmosphere, which then forms clouds and precipitation).

http://www.grida.no/climate/ipcc/regional/201.htm
Higher air temperatures could strongly influence the processes of evapotranspiration, precipitation as rain or snow, snow and ice accumulation, and melt—which, in turn, could affect soil moisture and groundwater conditions and the amount and timing of runoff in the mid- and high-latitude regions of North America. Higher winter temperatures in snow-covered regions of North America could shorten the duration of the snow-cover season. In mountainous regions, particularly at mid-elevations, warming could lead to a long-term reduction in peak snow-water equivalent, with the snowpack building later and melting sooner (Cooley, 1990). Glacial meltwater also is a significant source of water for streams and rivers in some mountainous regions, with the highest flows occurring in early or midsummer (depending on latitude). For example, glacial meltwater contributes an average of 85% of the August flow in the Mistaya River near Banff, Alberta (Prowse, 1997). Accelerated glacier melt caused by temperature increases means more runoff in the short term, but loss of glaciers could result in streams without significant summer flow in the future (IPCC 1996, WG II, Sections 7.4.2 and 10.3.7). Late-summer stream discharge could decrease suddenly within only a few years. A steady pattern of glacial retreat is apparent in the southern Rocky Mountains below central British Columbia and Alberta. Water supplies in small communities, irrigation, hydroelectric generation, tourism, and fish habitat could be negatively impacted (IPCC 1996, WG II, Chapter 7; Brugman et al., 1997; Prowse, 1997).