The risks of global warming are already pressing upon Earth's natural systems and, to a lesser extent, humanity, and research undertaken since 2007 is yielding sober warnings about greater frequency and intensity of extreme weather events; hits to farm production in many areas, especially in developing countries; and dramatic changes to ecosystems, including extensive loss of species.
That assessment is contained in a new report from the Intergovernmental Panel on Climate Change (IPCC), released Monday, which includes plenty of examples of global warming's already-observed effects.
"We have assessed impacts as they are happening on natural and human systems on all continents," said IPCC Chairman Rajendra Pachauri during a briefing Monday. "In view of these impacts, and those that we have projected for the future, nobody on this planet is going to be untouched by the impacts of climate change."
The volume that Working Group 2 produced involved nearly 400 authors who reviewed some 12,000 studies, according to Mr. Pachauri.
But unlike past IPCC volumes that focused on impacts, adaptation, and vulnerability, this latest report on these subjects carries what Christopher Field, co-chair of Working Group 2, calls "guarded optimism" about the world's ability to confront the risks of climate change.
"We have a deep pool of options, we have a deep pool of experience, and I think a deep pool of creativity and innovation" that can be applied to managing the risks from global warming, he said during a briefing on the report, which governments approved at an IPCC meeting in Yokohama, Japan, March 25-29.
"What you see in the report is a big emphasis on how adaptation, vulnerability reduction, and building resilience can be key parts" of blunting the effects of global warming, he said, even as countries continue to push for reductions in greenhouse-gas emissions in hopes of minimizing the effects of global warming.
The report's extensive focus on reducing vulnerability to climate change, particularly in its "Summary for Policymakers," makes it "the most actionable of any IPCC report I've yet seen," says Peter Frumhoff, a visiting professor at Stanford University and the director of science and policy at the Union of Concerned Scientists, a group focusing on environmental, energy, and nuclear security issues based in Cambridge, Mass.
"It really provides an exceptionally sober, thoughtful appraisal of what we know about managing our way through the uncertainties and growing risks that climate will impose on the world," he says.
Unlike the volume that Working Group 2 published in 2007 as part of the IPCC's periodic review of climate science, this volume more-explicitly acknowledges that climate change is a significant player, but that it's not the only one determining the level of risk society faces from continued warming, at least in the short term.
For example, for key economic sectors, the report notes that demographic changes, income levels, technology, prices for goods and services, regulations, and approaches to governing "are projected to be large relative to the impacts of climate change."
When considering the effects of global warming on public health, the report notes that for the near term the most effective ways to reduce people's vulnerability are to improve access to clean water and sanitation, to provide basic health care, including child-health services, to boost governments' ability to help people prepare for natural hazards and to provide timely help when natural disasters strike.
The greater consideration of factors other than climate that play a key role in determining a country's or community's vulnerability to global warming is a welcome change, suggests Netra Chhetri, a senior sustainability scientist at Arizona State University's Global Institute of Sustainability in Tempe.
"We have to be really, really careful how much we want to load things on climate," says Dr. Chhetri, a contributing author to the report's chapter on food security.
This time around, he says, the IPCC is acknowledging "that non-climate stresses are very, very important; they come from politics, they come from socio-economic inequalities, they come from cultural differences," and that these must be addressed as well as greenhouse-gas emissions or a need for higher sea walls.
Although the report places a higher emphasis than in the past on opportunities for adaptation and increasing resilience, it also points to examples of global warming's impact today, as well as sobering projections about what humanity can expect with and without curbs on greenhouse-gas emissions.
For instance, studies show that global warming is undermining crop yields for wheat and maize globally, although in the aggregate, rice and soybean yields remain unchanged. A handful of studies show some positive effects from climate change on agriculture at high latitudes, but the panel notes that additional work needs to be done to determine if the impact overall is positive or not. Generally, however, negative effects on crops are more common than positive effects.
Changes in rain and snowfall patterns and melting glaciers are reducing water availability and quality in many regions, the report notes.
Affects on human health to this point are harder to quantify, according to the panel. Global warming's contribution to ill health is tiny compared with non-climate factors. Still, the panel observes that some regions have seen a decline in the number of deaths related to exposure to cold, while the number of deaths attributed to heat have risen. And the distribution of some water-borne illnesses and the insects that spread them has expanded to new areas.
Overall, evidence for global warming's effect on the planet is most obvious in changes to natural systems, the panel observes.
As for the future, one concern centers on the rates of change projected if emissions follow projections built on plausible medium to high emission rates. If emissions follow any of those trajectories, that presents a high risk of abrupt, irreversible changes to regional ecosystems and the services they provide, including the sequestration of carbon dioxide and methane.
The panel also reports that global warming over the course of the century will prompt more people to migrate from areas where climate change has made farming untenable – especially where farming relies exclusively on rainfall for irrigation. In some cases, however, that mobility can reduce the vulnerability of the people on the move, the report adds. In any case, the panel didn't give a number for people expected to be displaced by climate, because migration has multiple triggers and the panel had low confidence in the few studies that tried to make such projections.
Ocean acidification and warming are expected to alter marine ecosystems in ways that reduce the availability of fish in several regions, a process that already has started in some parts of the tropical oceans, the panel notes. The changes come on top of the erosion of food security from global warming's effect on farming.