Thresholds

Policy makers and the general public alike now largely accept that the Arctic, Antarctica and many mountain regions already have warmed two-three times faster than the rest of the planet.  What is less understood, outside the scientific community, is that the very nature of the cryosphere – regions of snow and ice – carries dynamics that once triggered, in some cases cannot be reversed, even with a return to lower temperatures or CO2 levels.  “Thresholds and Closing Windows: Risks of Irreversible Climate Change,” seeks to convey a message from IPCC AR5 and key cryosphere research since then: that at current commitment levels or INDCs, humanity faces very high risk of crossing certain irreversible thresholds in its cryosphere regions – setting into motion changes that cannot be stopped or reversed, in some cases not without a new “Little Ice Age”, and perhaps not even then. The only way to prevent these dynamics from beginning is to make sure temperatures never rise that high.

Read the report here (PDF)

In connection with the release of the print version of “Thresholds” at the Paris climate talks on December 9, a group of the Report reviewers and other leading scientists called for greater reductions to avoid crossing these dangerous thresholds in “cryosphere” – snow and ice – regions, taking out an ad in The Guardian newspaper to express their concern. “This can set into motion very long-term changes that cannot be stopped or reversed, even if temperatures later decrease,” they note. “Some changes, such as committed sea-level rise from the great polar ice sheets, cannot be reversed short of a new Ice Age.”

The 23 scientists, a majority of them IPCC AR5 authors, called on pledges leading to temperatures under two degrees – and preferably, under 1.5 degrees over pre-industrial for the best chance of limiting these risks. Click here to see the “Losses on All Human Timescales” call.

Presentations:

Introduction to Cryosphere Risks (.ppt) *Please note .pptx files cannot be opened in Firefox. You must use “Save As” or Google Chrome to view these presentations.

Anders Levermann – Ice Sheets (large .ppt)

George Kaser – Mountain Glaciers (.pptx) *Please note .pptx files cannot be opened in Firefox. You must use “Save As” or Google Chrome to view these presentations.

Susan Natali – Permafrost (.pptx) *Please note .pptx files cannot be opened in Firefox. You must use “Save As” or Google Chrome to view these presentations.

Dirk Notz – Arctic Sea Ice (.pdf)