Climate Change Uncertainty For Next Few Decades

Earlier this month, there was a bit of reporting on a draft of a new study, Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX), by Intergovernmental Panel on Climate Change (IPCC) dealing with future trends in extreme weather. After seeing a leaked version of the SREX's Summary for Policymakers, the AFP reported: 

A new UN report concludes that man-made climate change has boosted the frequency or intensity of heat waves, wildfires, floods and cyclones and that such disasters are likely to multiply in the future…

– It is "virtually certain"—99-100% sure—that the frequency and magnitude of record-hot days will increase over the 21st century on a global scale.

– It is "very likely" (90-100% certainty) that the length, frequency and/or intensity of warm spells, including heat waves, will continue to increase over most land areas.

– Peak temperatures are "likely" (66-100% certainty) to increase—compared to the late 20th century—up to 3.0 degrees Celsius (5.4 degrees Fahrenheit) by 2050, and 5.0 C (9.0 F) by 2100.

– Heavy rain and snowfall is likely to increase, especially in the tropics and at high latitudes.

– At the same time, droughts will likely intensify in the Mediterranean region, central Europe, North America, northeastern Brazil and southern Africa.

– Rising and warming seas are also very likely to boost the destructive power of cyclones, while melting glaciers and permafrost, along with heavier precipitation, will trigger more landslides.

Now, BBC reporter Richard Black has apparently seen a more recent draft version that acknowledges a lot of scientific uncertainty about such predictions. He reports:  

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 new report is scheduled for release this Friday, so it will be interesting to see if the IPCC highlights or downplays the scientific uncertainties.