The world's forests are being damaged by climate change-related heat and drought, even in areas not traditionally known for water shortages, U.S. Geological Survey researchers say in the first global assessment of tree deaths from heat stress and drought.
The findings highlight the very real risk that tree mortality could become a bigger problem as global climate change progresses.
They also suggest that emissions offset programs designed to prevent logging and clear cutting of forests are missing the big picture: Allowing polluters to pay to preserve or plant trees rather than reduce their own greenhouse gas emissions may keep one area of trees alive, but it continues to endanger forests around the world.
The study's authors reviewed reports on 88 recent episodes of widespread tree deaths.
Although mass tree deaths aren't always related to global warming, the findings suggest that "at least some of the world's forested ecosystems already may be responding to climate change."
"The widespread examples of drought and heat-induced tree mortality that we document illustrate how climate can drive abrupt, broad-scale impacts to essential forest services ranging from timber and protection of watersheds and biodiversity to recreational, aesthetic and spiritual benefits," said Craig D. Allen, a USGS scientist who led the study.
Simply planting more trees will not solve the problem, because it's not a simple one-for-one equation.
"Trees can die much more quickly than they grow," Allen points out. As the study notes, severe drought can kill a 200-year-old tree within months.
The number of documented episodes of tree loss related to warming and droughts has also risen rapidly in recently years, the study says. With scientists' knowledge today of the connection between greenhouse gas emissions and global warming, the findings are "a sign that far greater chronic forest stress and mortality risk should be expected in coming decades."
The study cites multiple examples of climate change-related tree loss over the past 30 years on every forested continent:
• In Africa, studies have linked heat and drought to widespread tree deaths in the tropical moist forests in Uganda, as well as mountain acacia in Zimbabwe, mesic savanna trees in South Africa's Kruger National Park, and centuries-old Aloe dichotoma in Namibia.
• In Asia, severe droughts have been associated tree of several species dying in the tropical dry forests of western India. Recent droughts have killed pine trees across more than 2,000 square miles of China. In Russia, forestry officials have mapped out nearly 300,000 square miles of forest consider at high-risk.
• In Australia, long droughts have killed Eucalyptus and Corymbia. Even New Zealand's thick forests, made famous by the Lord of the Rings movies, have not been immune.
• In Europe, studies document hot and dry conditions over the past two decades killing trees in the Mediterranean region, including oak, beech and pine species in France. Even Greece's most drought-tolerant pines species have suffered.
• In North America, the northern of spread of insects with the warming climate has affected 77,000 square miles of forest from Mexico to Alaska, hitting ash, oak and pine particularly hard.
• In South America, a hot and severe drought in 2005 across the Amazon basin, linked to an abnormally warm North Atlantic, was blamed for an extensive deaths of trees, "indicating vulnerability of Amazonian forests to moisture stress."
Given that this is just the first global assessment of the links between tree mortality and climate change, more study is needed. The authors note that current models for projecting tree mortality don't take these new influences into account, and they say no one is tracking climate-driven tree deaths on a global basis.
However, the findings clearly point to the need for better protection of all forests, not merely those that are in logging areas or drought-prone regions.
The majority of forestry carbon projects at the moment address forests along the equator, where deforestation occurs at a rate of 32 million acres per year. If we begin to take into account not only the logging of forests but also climate change-related tree deaths, there are many more forests all over the world that are in danger.
The study's findings also underscore an issue that many environmentalists have with the vast number of offsets allowed under a cap-and-trade proposal in U.S. climate legislation. Those who oppose cap-and-trade argue that it will simply allow companies to pollute so long as they're paying to, for example, plant trees in Brazil, whereas a carbon tax would be a stronger financial incentive to reduce carbon emissions.
A recent Greenpeace report gave the example of a forest preservation project in Bolivia funded by oil and utility interests that overestimated its carbon absorption potential by 900 percent. In 1997, the project's creators estimated its emissions reduction capacity at 55 million metric tons of CO2; by 2009, they had lowered that estimate to 5.8 million.
A video by two EPA lawyers illustrates another problem with forestry offsets with the following example:
"I pay you not to cut down an acre of your forest so that it can continue to capture carbon," Allan Zabel explains. "Then I get to keep emitting carbon at my coal-fired power plant.
"But maybe you weren't planning to cut down your forest and you are receiving a bonus for doing something you were going to do anyway. Even if you were planning to cut your forest and now you won't, demand for the wood doesn't go away so it's likely that a different forest will be cut. In either case, there is no reduction in greenhouse gases.
"The only result is an increase in greenhouse gas emissions from my burning extra coal."
So far, negotiators involved in the international climate talks haven't provided much assurance of accountability for deforestation offset programs. At recent talks in Barcelona, advocates pushed for language to prevent forests being razed and replaced by tree plantations under REDD, but the draft that emerged no longer included opposition to such practices and instead promised to promote actions that "do not provide incentives for conversion of natural forests."