Global warming will affect societies around the world through more prolonged droughts, more intense rains and flooding, changes in the timing of seasonal rainfall and snowmelt, and a projected increase in the spread of animal- and insect-borne diseases, scientists say.
But it will affect plant and animal species even more dramatically. A shift in climate zones could lead to extinction of some species and the spread of others, according to a report set to be released Friday by the Intergovernmental Panel on Climate Change.
In turn, many of these ecological shifts will affect humans, writes Chris Field, founding director of the Carnegie Institution's department of global ecology at Stanford University, in an e-mail from the IPCC talks in Brussels. "A large fraction of the impacts of climate change on people are transmitted through ecosystems."
If the average temperature rises by 1.8 degrees C (3.2 degrees F.) by the end of this century – the low end of the IPCC's projected range – it would still be possible to set up preserves and maintain almost all of the planet's major ecosystems and the species they contain, says Camille Parmesan, a biologist at the University of Texas in Austin. But if temperatures rise much higher than that, "We're going into a realm Earth hasn't seen for a very long time. Most of the species we have on Earth did not evolve under that warmer climate."
Already, the global climate has warmed an average of 0.7 degrees C during the 20th century, she notes. So far, the ecological changes have been relatively benign.
What's worrisome is if heat-trapping greenhouse-gas emissions continue to accumulate under the IPCC's "business as usual" scenario, says John Williams, who studies plant dynamics at the University of Wisconsin at Madison. If that happens, up to 48 percent of Earth's land surface will lose existing climate zones, he and two colleagues recently calculated.
That loss is at the cold ends of the spectrum – largely toward the poles and at high altitudes in the tropics. At the same time, some 12 to 39 percent of the planet will see hotter – or what the team calls "novel" – climate zones.
How novel? The prolonged drought the US Southwest has experienced is probably not temporary, says an international team of scientists. Instead, it's likely to be a manifestation of the northward expansion of a belt of subtropical, dry climate conditions. If the team's projections are correct, the Southwest's average climate conditions will be just as dry as the drought or the Dust Bowl. The team's results appear in Friday's issue of the journal Science's online service.
Not all effects will be harmful, many experts agree. High latitudes would experience a longer growing season. And increased carbon-dioxide concentrations in the atmosphere have stimulated plant growth worldwide – at least for now.
But such effects have their limits. Researchers have found that if water and soil nutrients available to trees, shrubs, and grasses don't keep pace with rising CO2, plant growth will stall as plants get too much of a usually good thing.
Even when plants take up lots of CO2 in the spring, they throttle back in the summer if conditions heat up and dry out. Such conditions are expected to cover broad swaths of Earth's landscape. This essentially cancels the effects of the spring uptake.
There may be other, more subtle effects. Researchers at the University of New Hampshire, for example, have found that trees, which emit hydrocarbons naturally, give off more hydrocarbons as CO2 levels rise. Scientists already have shown that these hydrocarbons, when mixed with nitrogen-based gases from coal-fired power plants, can contribute smog and tiny aerosol particles to already polluted urban air. The New Hampshire group is now trying to see what impact these additional hydrocarbons may have on air quality.
In general, "With the small climate change we've already had, 59 percent of the world's species have shown a response – on all continents, in all major oceans, and across taxonomic groups," says Dr. Parmesan. Some 70 species have become extinct, largely at the poles or on mountaintops, where cold-adapted organisms have no place left to go as warmth creeps into higher latitudes and altitudes. Others, such as polar bears, are threatened. In other cases, rising CO2 levels are forcing organisms into a new regime where they have to compete for resources. In California, for example, researchers have found that higher CO2 levels have delayed flowering in some wild grasses but accelerated it in wild herbs that share the same turf. This is forcing the plants to compete directly for nutrients – ultimately changing the character of the ecosystem.
In the oceans, researchers are noting shifts in fish migration patterns. Coral-reef bleaching remains a concern, but its connection to global warming is still somewhat contentious, notes Chris Langdon, a marine scientist at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. Measurements don't go back far enough in time to see if bleaching events are any more frequent than in the past.
But ocean acidification, a byproduct of rising CO2 levels in the air, is far more straightforward. It threatens a range of organisms – from tiny shell-forming plankton to reefs themselves. Some researchers say the changes in ocean chemistry from human CO2 emissions are expected to last from tens of thousands to hundreds of thousands of years, until natural buffering can take hold.
With some care and feeding, however, some ecosystems may be able to retain their resilience in the face of changes expected from global warming. Researchers at Duke University have found, for example, that if rates of sea-level rise remain relatively low, tidal marshes build themselves to keep pace, continuing to provide a buffer against storm surge and serving as a nursery for marine life.
"Marshes are not necessarily doomed," says Matt Kirwan, who led the study.