Excitement! The draft of the next IPCC report fell off the back of a truck in front of the BBC's Richard Black

He advises that it is a lot more cautious than their previous efforts

On the one hand, it says it is "very likely" that the incidence of cold days and nights has gone down and the incidence of warm days and nights has risen globally. And the human and financial toll of extreme weather events has risen.

But when you get down to specifics, the academic consensus is far less certain.

There is "low confidence" that tropical cyclones have become more frequent, "limited-to-medium evidence available" to assess whether climatic factors have changed the frequency of floods, and "low confidence" on a global scale even on whether the frequency has risen or fallen.

In terms of attribution of trends to rising greenhouse gas concentrations, the uncertainties continue.

While it is "likely" that anthropogenic influences are behind the changes in cold days and warm days, there is only "medium confidence" that they are behind changes in extreme rainfall events, and "low confidence" in attributing any changes in tropical cyclone activity to greenhouse gas emissions or anything else humanity has done.

(These terms have specific meanings in IPCC-speak, with "very likely" meaning 90-100% and "likely" 66-100%, for example.)

And for the future, the draft gives even less succour to those seeking here a new mandate for urgent action on greenhouse gas emissions, declaring: "Uncertainty in the sign of projected changes in climate extremes over the coming two to three decades is relatively large because climate change signals are expected to be relatively small compared to natural climate variability".

It's also explicit in laying out that the rise in impacts we've seen from extreme weather events cannot be laid at the door of greenhouse gas emissions: "Increasing exposure of people and economic assets is the major cause of the long-term changes in economic disaster losses (high confidence).

The succour only lasts for so long, however.

If the century progresses without restraints on greenhouse gas emissions, their impacts will come to dominate, it forecasts:

"It is very likely that the length, frequency and/or intensity of warm spells, including heat waves, will continue to increase over most land areas...

"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...

"Mean tropical cyclone maximum wind speed is likely to increase...

"There is medium confidence that droughts will intensify in the 21st Century in some seasons and areas...

"Low-probability high-impact changes associated with the crossing of poorly understood thresholds cannot be excluded, given the transient and complex nature of the climate system."

The draft report makes clear that lack of evidence or lack of confidence on a particular impact doesn't mean it won't occur; just that it's hard to tell.

It's impossible to read the draft without coming away with the impression that with or without anthropogenic climate change, extreme weather impacts are going to be felt more and more, simply because there are more and more people on planet Earth - particularly in the swelling "megacities" of the developing world that overwhelmingly lie on the coast or on big rivers close to the coast.


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