March 31, 2014 11:47 p.m. ET
Climate change is having a big impact on both the earth's natural systems and how people live, according to the most comprehensive assessment of the threat of a warming planet done so far.
The second part in a four-part report by the U.N. Intergovernmental Panel on Climate Change warns that the problem will become increasingly difficult to manage, with possible threats to everything from the food supply to coral reefs and low-lying coastal areas.
"The striking feature of observed impacts is that they are occurring from the tropics to the poles, from small islands to large continents, and from the wealthiest countries to the poorest," said Chris Field, a researcher at the Carnegie Institution of Science in Stanford, Calif., and one of the lead authors of the report.
The report is the most comprehensive assessment of its kind since a similar IPCC report was published in 2007. The first part, which analyzed the scientific evidence of climate change and was released in September, concluded that human activity was the dominant cause behind a rise in global temperatures.
Put together by more than 300 scientific authors, the report is based on hundreds of additional studies whose findings weren't available for the 2007 report. While many of the conclusions echo those in the previous one, they are based on many more measurements – taken from the seas, land and from space.
The authors acknowledge that it is still hard to pinpoint the impact of climate regions in many parts of the world. Future projections are also hindered by a reliance on highly-complex models, which are only as good as their design and the quality of data that goes into them. For this reason, some of the IPCC's projections in the past have been criticized as being alarmist.
Some of the most striking findings involved the impact on crop and food production. For example, observed data and several studies showed that a warming climate generally reduces yields of staple cereals such as wheat, rice and maize, though the impact varies in different regions and latitudes. That could leave many more people struggling to feed themselves.
The report noted, with "medium confidence," that since 2007 there had been several periods of rapid food and cereal price increases following climate extremes in key producing regions, and that "several of these climate extremes were made more likely as the result of [man-made] emissions."
"The key new finding that they're emphasizing is that climate change is being felt by people and not just by polar bears," said Myles Allen, climate scientist at the University of Oxford in England, who wasn't involved in the latest IPCC report.
Since the last impact-assessment done by the IPCC, "the scientific literature has advanced and we have a lot more information, especially about the opportunities for adapting to climate change."
Not everyone agreed with the report. A body known as the Nongovernmental International Panel on Climate Change on Monday released a 1,062-page report citing studies done in the peer-reviewed literature and came to different conclusions. Its analysis found that higher carbon dioxide concentrations and rising temperatures are causing "no net harm to the global environment or to human health and often finds the opposite: net benefits to plants, including important food crops, and to animals and human health."
The nongovernmental panel is a group of scientists, backed by the libertarian think tank, The Heartland Institute in Chicago, who first came together in 2003 to provide an independent review of the climate science cited by the IPCC. Heartland doesn't accept government funding. According to its website, approximately 8,300 supporters contribute to an annual budget of $6 million.
While many scientists who back the IPCC findings agree that rising CO2 levels and warmer climes will prove beneficial to some crops and plants in certain regions, they argue that the net harm will be greater than any benefits if temperatures and CO2 levels continue to rise.
As in past assessments, the IPCC has qualified the degree of certainty for each key finding on the basis of different levels of confidence, ranging from "very low" to "very high." The levels are determined by a team of researchers who together sift through scores of studies on a particular subject to evaluate the evidence. "Generally, evidence is more robust when there are multiple, consistent lines of high-quality evidence," the IPCC said.
Based on an assessment of several studies they cite, the authors conclude they have "high confidence" that mass bleaching and death of coral, triggered by unusually warm temperatures, "is the most widespread and conspicuous impact of climate change."
Elsewhere, the study says it "is very likely that mean annual temperature has increased over the past century over most of North America," while warming has been less pronounced over areas of the central and southeastern U.S. In this case, "very likely" means 90%-100% probability, according to the IPCC's methodology.
The IPCC report says a global-temperature rise of 2.5 degrees Celsius above preindustrial levels could lead to global economic losses between 0.2% and 2.0% of income.
Funding needed to offset the impact of climate change is lacking, the report warned, saying developing countries would need between $70 billion to $100 billion a year to implement needed measures. And efforts to reduce the effects of climate change would only have a marginal effect on reducing poverty unless "structural inequalities are addressed and needs for equity among poor and non-poor people are met."
The third installment of the IPCC assessment, on how to cut greenhouse gases, will be published in April.
—Alexander Martin in Tokyo contributed to this article.
