Heavier rainfall, fiercer storms and intensifying droughts are likely to strike the world in the coming decades as climate change takes effect, the world's leading climate scientists said on Friday.
Rising sea levels will increase the vulnerability of coastal areas, and the increase in "extreme weather events" will wipe billions off national economies and destroy lives, according to the Intergovernmental Panel on Climate Change (IPCC), the body of the world's leading climate scientists convened by the United Nations.
Scientists have warned of these effects for years, but yesterday's report – the "special report on extreme weather" compiled over two years by 220 scientists – is the first comprehensive examination of scientific knowledge on the subject, in an attempt to produce a definitive judgment. The report contained stark warnings for developing countries in particular, which are likely to be worst afflicted in part because of their geography, but also because they are less well prepared for extreme weather in their infrastructure and have less economic resilience than developed nations. But the developed world will not be unscathed – heavier bursts of rainfall, heatwaves and droughts are all likely to take their toll.
Chris Field, co-chair of the IPCC working group that produced the report, said the message was clear – extreme weather events were more likely. "Some important extremes have changed and will change more in the future. There is clear and solid evidence [of this]. We also know much more about the causes of disaster losses."
He urged governments to take note – many of the economic and human impacts of disasters can be avoided if prompt action is taken: "We are losing way too many lives and economic assets in disasters."
The report was timed just before crucial talks taking place later this month in Durban, South Africa, where the world's governments will discuss a new global agreement to tackle greenhouse gas emissions and climate change.
Europe's climate chief, Connie Hedegaard, said the report should galvanise governments to act, especially when added to the stark warnings last week from the International Energy Agency that the world has only five years to take the emissions-cutting measures needed to prevent catastrophic global warming. She said: "Last week, the serious warnings from the International Energy Agency. Today, this IPCC report … With all the knowledge and rational arguments in favour of urgent climate action, it is frustrating to see that some governments do not show the political will to act. In light of the even more compelling facts, the question has to be put to those governments in favour of postponing decisions: for how long can you defend your inaction?''
Bob Ward, policy and communications director at the Grantham Research Institute at the London School of Economics, said the report meant the science was now clear: "This expert review of the latest available scientific evidence clearly shows that climate change is already having an impact in many parts of the world on the frequency, severity and location of extreme weather events, such as heatwaves, droughts and flash floods.
"This is remarkable because extreme events are rare and it is difficult to detect statistically significant trends in such small sets of data. What is more, these trends have been identified over the last few decades when the rise in global average temperature has been just a few tenths of a centigrade degree.
"The report shows that if we do not stop the current steep rise in atmospheric levels of greenhouse gases, we will see much more warming and dramatic changes in extreme weather that are likely to overwhelm any attempts human populations might make to adapt to their impacts."
But the summary report was also hedged with caveats, reflecting the difficulty in tying specific extreme weather events to human-induced global warming. Attributing economic losses – such as the damage from storms and floods – is also tricky, because of other factors involved. Rising urbanisation and wealth mean that losses today are higher than in the past.
This point is likely to become particularly contentious in the future, as developed country governments are called upon to provide funding to the poor world to help people adapt to the effects of climate change.
Although the scientists said they were still unsure whether a warming climate would result in an increase in the frequency of hurricanes and other tropical cyclones, there was a stark warning for the northern hemisphere, and areas of Europe and North America where currently hurricanes hardly ever happen. There has been a "poleward shift" in the pattern of the storms, which will mean severe storms are more likely to strike areas such as New York and the Atlantic coast of Europe.
Scientific models also show that it is "very likely" – a term that denotes, in IPCC parlance, a 90% to 100% probability – that the "length, frequency and/or intensity of warm spells or heat waves will increase over most land areas". This means that record hot days, which previously could be expected once in 20 years, are now likely every other year. This could have a serious impact on old people and the very young in particular, who are more vulnerable to changes in temperature.
The report said: "It is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy falls will increase in the 21st century over many areas of the globe. This is particularly the case in the high latitudes and tropical regions, and in winter in the northern mid-latitudes. Heavy rainfalls associated with tropical cyclones are likely to increase with continued warming." This means that cloudbursts that could have been expected once in 20 years will now become a one-in-five-year occurrence. The scientists were reluctant to translate this into concrete warnings over the frequency of floods, because floods depend on local factors such as topography, but said floods, mudslides and landslips are associated with stronger rainfall punctuated by drier spells.
The scientists said there was "medium confidence" that "droughts will intensify in the 21st century in some seasons and areas, due to reduced precipitation and/or increased evapotranspiration". They pinpointed the most vulnerable areas as southern Europe and the Mediterranean region, central Europe, central North America, Central America and Mexico, north-east Brazil, and southern Africa.
Simon Brown, climate extremes research manager at the Hadley Centre, the climate research unit of the UK's Met Office, said: "This focus of the IPCC on extremes is very welcome as less emphasis has traditionally been given to these phenomena which are very likely to be the means by which ordinary people first experience climate change. Human susceptibility to weather mainly arises through extreme weather events so it is appropriate that we focus on these which, should they change for the worse, would have wide-ranging and significant consequences. This review will be very helpful in progressing the science by bringing together a wide range of studies – not just on the physical weather aspects of climate extremes but also on how we might adapt and respond to their changes in the future."
Development campaigners urged swift action from governments meeting at the end of this month in Durban, South Africa, to continue negotiations on a global agreement to tackle climate change. Tim Gore, Oxfam climate change adviser, said: "[This] is a warning bell for world leaders to act now on climate change to save lives and money. The link between climate change and an increase in the frequency and intensity of some extreme weather events is becoming ever clearer, and it is the world's poorest and most vulnerable people who are being hit the worst. Floods and droughts like those which recently hit east Asia and the Horn of Africa can wipe out whole harvests, contributing to soaring food prices and driving poor people into hunger."
He added: "Estimates suggest that every dollar invested in adaptation to climate change could save $60 in damages. Governments must find the new money needed to invest now, and avoid the far higher costs of clean-up and lives lost later."