What we’ve learned about climate change in 2008

There is a great overview now available in Nature Reports Climate Change, which discusses how far our scientific understanding of climate change has come in the last 12 months. It is available free online or for PDF download. Written for the non-specialist, it is readily intelligible (much in the New Scientist style), but goes into more detail than you’d find in any newspaper article.

The top 5 things we’ve learned are [my short version]:

1. Other greenhouse gases are also worrying [including methane, which has started to rise again, and even gases such as nitrogen trifluoride, though the extent of this as a problem remains quite controversial]

2. Arctic summer sea ice is in rapid decline [2nd lowest extent on record after 2007, and perhaps more importantly, the lowest volume of ice ever – this, along with the methane increase, could indicate a tipping element has been crossed that will kick climate change into a higher gear]

3. Warming is already having an impact [biological and geophysical signals are increasingly apparent yet they don’t use thermometers, relocation of species may eventually become necessary to avoid extinctions]

4. The hockey stick holds up [Mann et al. published a new compilation of proxies in PNAS, which extended the length of the stick and confirmed the original form of the reconstruction from 1998, except that the handle is now a little more twisted]

5. Sceptics are still out there [with particular commentary on political motivations and the conspiracy theory claims of a great global scam]

What we are still working on (the ‘uncertainties’):

1. How much warming and by when [2008 was the hottest La Nina year on record, but attempts at decadal-scale predictions clarify the role of natural variation in driving short-term cycles and suggest only moderate warming in the next 5 years. Yet the media’s interpretation of these findings has been strongly challenged, e.g. here and here]

2. Where to stabilize [550 ppm or 450 ppm CO2-e is no longer the appropriate target – Hansen’s <350 ppm is quickly gaining traction as the go-to goal to avoid “irreversible catastrophic effects”; others despair that it is practically impossible to avoid 650 ppm]

3. Where the missing carbon is going [carbon sinks are still hard to quantify precisely, and the whether the oceans are losing their ability to draw down CO2 remains equivocal (and distorted by the deniers); better monitoring and accounting systems are being developed to address this critical uncertainty]

4. Whether warming worsens storms [still very difficult to determine whether global warming will increase the overall frequency of intense storms (partly because these are difficult to resolve in current-generation climate models), but clear evidence has emerged of the increase in most intense category 4-5 hurricanes/cyclones/typhoons]

5. How fast Greenland is melting [melt on the last northern ice sheet, Greenland, is accelerating, and new estimates put its contribution to sea level rise 0.8 – 1.3m by 2100 (and that doesn’t count the extra contribution from West Antarctica) — accurately capturing the physics of ice sheet dynamics is a key area of research, especially how subsurface melting contributes to ice sheet slippage and bottom lubrication; when does Greenland pass the point of no return?]