Global Warming is Accelerating

Photo of a small waterfall and rocky, cascading stream in the White Mountains, New Hampshire

Scientists documented major changes to natural systems around the planet in the 2007 report of the Intergovernmental Panel on Climate Change (IPCC). New science published after the IPCC report has shown how global warming is happening faster than anticipated. Sea-level rise may be accelerating, Arctic sea ice is rapidly melting, and forests are being transformed.

Sea-level Rise Rates Larger Than Previously Projected

Sea-level rise as a consequence of global warming is a foregone conclusion. It only remains to be seen how much and how quickly. The 2007 IPCC report projected global average sea-level rise of 7 to 23 inches by the 2090s. This calculation did not take into account compelling new evidence of recent rapid melting in Greenland and Antarctica, that if continued could lead to sea-level rise of 5 or 6 feet this century (Read New Science Demonstrates Need for Aggressive Cap on Carbon Pollution ). That much sea-level rise would cause havoc for coastal communities and ecosystems, especially as population and development in coastal regions continue to grow.

A major challenge in projecting sea-level rise is the potential for destabilization and rapid melting of ice sheets, most notably the West Antarctic Ice Sheet (WAIS). Indeed, new data shows that Antarctica has been warming more than previously thought. Complete melting of the WAIS would cause 16 feet of sea level rise globally, and it would expose the United States to even more. A new study shows that if the WAIS collapses, the resulting changes in gravitational pull would cause water to slosh northward, adding another 4.3 feet of potential sea-level rise along northern continents.

Widespread Forest Mortality

A 2009 study by Philip van Mantgem and colleagues found that some unmanaged old-growth forests in western North America have recently experienced increased tree mortality due largely to warming and associated drought stress. New seedlings are not keeping pace with this loss of trees, foreshadowing sparser forests in the coming decades. Warmer and drier conditions are also conducive to widespread beetle and other insect infestations. For example, recent reports estimate that more than 32 million acres in western Canada have been decimated by mountain pine beetle infestations since 1999.

It is not surprising that higher temperatures and tree mortality have led to a rapid increase in western wildfires. Warmer springs and longer summer dry periods are linked to a 4-fold increase in the number of major wildfires and a 6-fold increase in the area of forest burned, compared with wildfires between 1970 and 1986.

The loss of these forests not only stops their conversion of carbon dioxide to oxygen and wood products, but also releases the huge quantities of carbon they store into the atmosphere. Forest fires in the United States already release about 80 million tons of carbon to the atmosphere every year.

We must find ways to restore our forests and maintain their ability to sequester huge quantities of carbon--even as global warming makes it more challenging.

Arctic Sea Ice Rapidly Melting

In 2007 scientists reported an alarming record low in late-summer Arctic ice. That September the remaining ice was some 39 percent less in area than the average observed from 1979 to 2000. The rapid melting of Arctic ice has outpaced what was projected by models of even a couple years ago (see Figure 2), requiring that the models be revised. Just two years ago the Intergovernmental Panel on Climate Change (IPCC) projected that the Arctic could be ice free during the summer by as early as 2050, but perhaps not until after 2100. In marked contrast to these recent projections, some scientists now say that the Arctic could be ice free in late summer by as early as 2012.

Warning Signs of Triggering Rapid Warming

Some scientists are beginning to raise concerns that we may already be seeing early signs of triggering potential rapid warming. It is possible that global warming could lead to changes in the climate system which in turn cause even more warming. These so-called "positive climate feedbacks"--in particular, those associated with greenhouse gas releases from soils, tundra, or ocean sediments; sea-ice and ice sheet disintegration; and vegetation migration--could make the climate system warm twice as much over the long term than previously calculated by climate models. Furthermore, they could come into play much sooner and more quickly than previously thought.

Melting Permafrost Releasing Methane

Vast areas of permafrost across the northern hemisphere store more than double the amount of carbon already in the atmosphere.20 Scientists worry that warming in the Arctic--which is proceeding about twice as fast as the global average--will lead to extensive thawing of these frozen landscapes, releasing carbon dioxide and methane, a greenhouse gas about 20-25 times more potent than carbon dioxide. In fact, the permafrost is rapidly degrading. For example, the area of discontinuous permafrost in Canada has increased by a factor of 3 or 4 compared to what it was from 1941-1991. Release of methane from melting permafrost may be one reason that atmospheric methane is on the rise in recent years after staying more or less constant for more than a decade.

Katey Walter and colleagues recently observed a significant source of methane from melting permafrost at the bottom of northern lakes. While still only accounting for about 4 percent of the total methane source, the contribution from lakes is nonetheless worrisome because it could get much larger as warming continues. The lakes in Siberia and Alaska could release an estimated 49 billion tons of methane--10 times more than what is currently in the atmosphere--if they were to thaw completely, a process that could happen within 500 to 1000 years.

Release of Methane in Deep Ocean Ice

Trapped deep in the ocean floor are huge quantities of methane hydrates frozen in ice structures called clathrates. The total amount of marine methane hydrates could be as much as 100 to 2000 times the amount of methane currently found in the atmosphere. If the ocean floor were to warm sufficiently, causing melting or increased underwater landslides, some of this gas could be released and bubble up to the atmosphere, leading to more global warming. A recent survey of the ocean waters above the East Siberian Arctic Shelf may have located the first signs of such methane release. Igor Semiletov and his colleagues measured methane bubbling up at a rate 10 times faster than just a decade ago, contributing to spikes of methane in the atmosphere up to 4 times greater than the global average concentration.

The chances of releasing a large amount of marine methane hydrates are low, and even if some methane were to be released it is still unknown how much would make it through the water column into the atmosphere. But the shear magnitude of the methane stored on the ocean floors--and the radical change in climate it could provoke--makes the risk of destabilization one we can not ignore. The real possibility of this is demonstrated by the geologic record. Destabilization of methane clathrates found on the ocean floor is suspected as a major contributor to rapid warming about 635 million years ago.

The Loss of Ice Reflectance

The ice covering the Arctic helps regulate our climate, a service that is being lost as more and more ice melts. Ice and snow reflect almost all the sunlight that hits it back to space. In marked contrast, the open ocean, which replaces the melting ice, absorbs nearly all the sunlight hitting it. Thus, as Arctic ice vanishes the region absorbs more of the Sun's energy, causing even more ice to melt in a powerful feedback. Some models show that once average annual temperatures in the Arctic rise to about 23°F or warmer, there will be an abrupt decline of sea ice and acceleration of warming.

Scientists have recently documented how the loss of sea ice translates directly into warmer temperatures as far as 900 miles inland. By looking at the historical record of seasonal sea-ice melt, David Lawrence and his collaborators found that episodes of rapid-sea ice loss were accompanied by warming over adjacent land at a rate 3.5 times more than the average warming expected. This means that sea-ice loss also contributes to thawing permafrost and the release of methane stored in Arctic soils.

The Declining Ability of the Ocean and Terrestrial Biosphere to Store Carbon

Over the last century, the ocean and the terrestrial biosphere have absorbed more than half of the fossil fuel CO2 emitted to the atmosphere, limiting the warming we have experienced so far. But, these carbon sinks are becoming less efficient. The ocean's ability to absorb additional CO2 declines as it becomes more and more saturated with it. In fact, the rate of CO2 absorption by the Southern Ocean has decreased by about 35 percent since 198131 and that of the North Atlantic decreased by 50 percent from 1995 to 2005. If emissions continue on a business-as-usual track, the declining ability of the oceans and land to absorb CO2 will translate into additional warming of more than 1.8°F over the next century.

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