Global warming is increasing the frequency and ferocity of some extreme-weather events, highlighting the need for governments at all levels to reduce vulnerabilities and increase the resilience of their citizens to such events.
The spectrum of tools available to help is familiar: improved forecasts and warnings for severe weather, rigorously enforced zoning and building codes, and restoration of ecosystems that serve as buffers between people and river floods or coastal-storm surges, for instance.
The success of such efforts, particularly in developing countries, depends in part on reducing poverty and social inequalities that can deprive people of the help they need to prepare for and cope with severe weather, the report's authors say.
In managing the risks from disasters, current and future, "one of the key messages from the report is that opportunities need to be taken advantage of at every scale – the local scale, the national scale, and the international scale," said Dr. Field, who cochairs one of two of IPCC working groups that contributed to the report. A briefing about the report was held Friday in Kampala, Uganda, where the two working groups were holding a joint meeting.
The challenge in looking for trends in weather extremes is that the extremes still tend to be relatively rare, researchers say.
Still, over the past decade, the researchers say, additional events and improvements in the tools to analyze them have allowed researchers to identify patterns in some climate features with increasing confidence.
Already, global warming's fingerprints are evident in broad temperature and precipitation trends over the past 60 years, say scientists from the two working groups – one that focuses on climate science and one that focuses on assessing the effects and vulnerability.
Globally, the number of warm days and nights has grown, while the number of cold days and nights has decreased. In the United States, researchers have documented an increase in the number of high-temperature records set per decade and a decrease in the number of records lows set.
Also globally, the number of storms delivering a deluge rather than gentle showers has increased in more regions than those regions recording decreases in intense rain or snowfall – a sign that the warming atmosphere is holding more moisture.
Moreover, a warming climate has contributed to sea-level rise, the report says. This has led to an increase in incidents of extreme coastal flooding during storms.
Among the changes researchers have tracked, they're most confident in the conclusions drawn about the changes described above. The picture is more mixed for trends in tropical cyclone activity, droughts, and river floods globally.
Looking ahead, the report acknowledges that projecting further changes to extreme-weather patterns as the climate warms carries significant uncertainties – particularly over the next 20 to 30 years, "because climate-change signals are expected to be relatively small compared to natural climate variability" over that period.
"The largest uncertainty in extreme events and in their projection into the future is associated with small-scale phenomena, which have large impacts" locally, adds Thomas Stocker, a climate researcher at the University of Bern in Switzerland who cochairs the IPCC working group that focuses on climate science.
Still, the report projects with virtual certainty that if greenhouse-gas emissions continue unabated, temperature extremes will grow warmer and occur more frequently.
By the end of the century, expect heat waves that occur on average every 20 years now to take place every two years, Field says. Likewise, the number of storms delivering heavy precipitation by century's end is expected to grow. The storms include tropical cyclones as well as winter storms in the northern mid-latitudes.
A 1-in-20-year storm is expected to return as often as once every five years. The number of tropical cyclones globally is likely to either decrease in number or remain at today's levels, but the maximum wind speeds are likely to increase in some ocean basins. And rising sea levels will continue to increase the hazards from ocean storms, including flooding and coastal erosion.
In at least one sense, the anticipated effects of global warming on extreme weather aren't new.
But with a run of highly visible, record-snapping severe-weather events over the past several years – including severe floods and intense droughts in Australia and heat waves in Europe and Russia that led to thousands of casualties – "it's dawning on people that the way we're going to see a lot of climate change is through extremes" and not global averages, says Gerald Meehl, a climate researcher at the National Center for Atmospheric Research in Boulder, Colo., who was one of the new report's reviewers.
As that realization has come, cities and counties around the US – from Lewes, Del., to San Diego – have been working the effects of a warming climate into their plans for risk reduction and new infrastructure, says Brian Holland, climate program director for ICLEI - Local Governments for Sustainability, a nongovernmental organization that helps communities plan for the effects of global warming.
Many of the approaches represent what the report refers to as low-regrets measures – approaches that make sense whether extreme-weather trends are driven by natural climate shifts or global warming.
But low regrets don't necessarily mean low costs for countries where poverty reigns. One key issue that negotiators at UN-sponsored climate talks will discuss later this month in Durban, South Africa, involves financial aid to developing countries for adaptation to climate change.
At global climate talks in Copenhagen, Denmark, in 2009 and Cancún, Mexico, in 2010, developed countries agreed to provide $30 billion in "quick start" assistance by 2012 to help poor countries pay for adaptation measures and for technology to reduce their greenhouse-gas emissions. It was to be the first step in providing $100 billion a year for adaptation and mitigation by 2020.
So far, the developed world has pledged $28.1 billion toward the 2012 goal, according to an analysis by the World Resources Institute.