IPCC draft report claims that “climate change signals are expected to be relatively small compared to natural climate variability over the coming two to three decades”
[…] What can science really tell us about weather extremes?
While the vulnerable meet in Dhaka, the Intergovernmental Panel on Climate Change (IPCC) will be sitting down in Kampala to answer the question.
For almost a week, government delegates will pore over the summary of the IPCC’s latest report on extreme weather, with the lead scientific authors there as well. They’re scheduled to emerge on Friday with an agreed document.
The draft, which has found its way into my possession, contains a lot more unknowns than knowns.
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.
Human hand fingered?
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).
“Long-term trends in normalized economic disaster losses cannot be reliably attributed to natural or anthropogenic climate change.”
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.