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Global warming 'past the point of no return'

By Steve Connor, Science EditorPublished: 16 September 2005The Independent

A record loss of sea ice in the Arctic this summer has convinced scientists that the northern hemisphere may have crossed a critical threshold beyond which the climate may never recover. Scientists fear that the Arctic has now entered an irreversible phase of warming which will accelerate the loss of the polar sea ice that has helped to keep the climate stable for thousands of years.

They believe global warming is melting Arctic ice so rapidly that the region is beginning to absorb more heat from the sun, causing the ice to melt still further and so reinforcing a vicious cycle of melting and heating.

The greatest fear is that the Arctic has reached a "tipping point" beyond which nothing can reverse the continual loss of sea ice and with it the massive land glaciers of Greenland, which will raise sea levels dramatically.

Satellites monitoring the Arctic have found that the extent of the sea ice this August has reached its lowest monthly point on record, dipping an unprecedented 18.2 per cent below the long-term average.

Experts believe that such a loss of Arctic sea ice in summer has not occurred in hundreds and possibly thousands of years. It is the fourth year in a row that the sea ice in August has fallen below the monthly downward trend - a clear sign that melting has accelerated.
Scientists are now preparing to report a record loss of Arctic sea ice for September, when the surface area covered by the ice traditionally reaches its minimum extent at the end of the summer melting period.

Sea ice naturally melts in summer and reforms in winter but for the first time on record this annual rebound did not occur last winter when the ice of the Arctic failed to recover significantly.
Arctic specialists at the US National Snow and Ice Data Centre at Colorado University, who have documented the gradual loss of polar sea ice since 1978, believe that a more dramatic melt began about four years ago.

In September 2002 the sea ice coverage of the Arctic reached its lowest level in recorded history. Such lows have normally been followed the next year by a rebound to more normal levels, but this did not occur in the summers of either 2003 or 2004. This summer has been even worse. The surface area covered by sea ice was at a record monthly minimum for each of the summer months - June, July and now August.

Scientists analysing the latest satellite data for September - the traditional minimum extent for each summer - are preparing to announce a significant shift in the stability of the Arctic sea ice, the northern hemisphere's major "heat sink" that moderates climatic extremes.

"The changes we've seen in the Arctic over the past few decades are nothing short of remarkable," said Mark Serreze, one of the scientists at the Snow and Ice Data Centre who monitor Arctic sea ice.

continued... The Independent

Climate Change special report

New Scientist

A massive global increase in the number of strong hurricanes over the past 35 years is being blamed on global warming, by the most detailed study yet. The US scientists warn that Katrina-strength hurricanes could become the norm.Worldwide since the 1970s, there has been a near-doubling in the number of Category 4 and 5 storms - the strength that saw Hurricane Katrina do such damage to the US Gulf coastline late in August 2005. Peter Webster of the Georgia Institute of Technology, Atlanta, says the trend is global, has lasted over several decades and is connected to a steady worldwide increase in tropical sea temperatures. Because of all these factors, it is unlikely to be due to any known natural fluctuations in climate such as El Niño, the North Atlantic Oscillation or the Pacific Decadal Oscillation. "We can say with confidence that the trends in sea surface temperatures and hurricane intensity are connected to climate change," says Webster's co-author Judy Curry, also of the Georgia Institute of Technology. The team looked at the incidence of intense tropical storms and the study results are the strongest affirmation yet that Katrina-level hurricanes are becoming more frequent in a warmer world.Unnatural trendThe study finds there has been no general increase in the total number of hurricanes, which are called cyclones when they appear outside the Atlantic. Nor is there any evidence of the formation of the oft-predicted "super-hurricanes". The worst hurricane in any year is usually no stronger than in previous years during the study period. But the proportion of hurricanes reaching categories 4 or 5 - with wind speeds above 56 metres per second - has risen from 20% in the 1970s to 35% in the past decade. "This trend has lasted for more than 30 years now. So the chances of it being natural are fairly remote," says Greg Holland of the National Center for Atmospheric Research (NCAR) at Boulder, ColoradoMoreover, says Webster, natural fluctuations tend to be localised. "When the east Pacific warms, the west Pacific cools, for instance. But sea surface temperatures are rising throughout the tropics today." The surface waters in the tropical oceans are now around 0.5°C warmer during hurricane seasons than 35 years ago. Satellite eraHurricanes form when ocean temperatures rise above 26°C. "The fuel for hurricanes is water vapour evaporating from the ocean surface. It condenses in the air and releases heat, which drives the hurricane's intensity," says Webster. "The tendency to Katrina-like hurricanes is increasing," Holland says. Without the warmer sea-surface temperatures, "Katrina might only have been a category 2 or 3".All the data for sea surface temperatures and hurricane numbers and intensities come from satellite data. "We deliberately limited this study to the satellite era because of the known biases [in the data] before this period," says Webster.This is the third report in recent months highlighting the growing risk to life and property round the world from hurricanes and tornadoes. In June, NCAR's Kevin Trenberth reported a rising intensity of hurricanes in the North Atlantic. And in August, Kerry Emanuel of MIT found a 50% increase in thedestructive power of tropical storms in the past half century.

New Scientist