With the release of the Garnaut Climate Change Review’s draft report on Friday, it’s worth quickly revisiting the reason for all of this — the changing state of the climate system — and take a look at a few significant developments since the hand-down of the latest IPCC reports.
A topic that raised a few eyebrows in the scientific community late last year was the 2007 decline in Arctic sea ice extent — here are the long-term satellite derived record of the annual minimum in sea ice extent (i.e. ocean area with at least 15% ice cover), and minimum geographical sea ice extent for 2007 and the previous record of 2005:
Last year’s minimum quite clearly obliterated all previous records. Much as it would be a mistake – given the obvious year-to-year natural variability in the record — to conclude too much from the spread between just two data points, the 2007 minimum lies an awfully long and lonely way off the linear downtrend apparent throughout the prior record.
And while the relative contributions of natural and human-induced climatic factors to the 2007 minimum remain an open question, what’s worrying is that – since the eventual loss of the Arctic sea ice will not be a linear process — its late stages might not look altogether dissimilar to the above. Senior scientist Mark Serreze at the National Snow and Ice Data Centre (NSIDC — here):
The sea ice cover is in a downward spiral and may have passed the point of no return. As the years go by, we are losing more and more ice in summer, and growing back less and less ice in winter. We may well see an ice-free Arctic Ocean in summer within our lifetimes.
All of which makes the approaching 2008 minimum a topic of keen interest (not to mention speculation – go here if you want a flutter). A low summer minimum tends to be preceded by poor winter ice growth, thin spring ice and significant polynya formation, all of which is consistent with this year’s observations so far. Here’s this year’s ice extent (relative to last year and the long term average):
Of course since it’s already in the ocean, the melting of sea ice doesn’t itself drive changes in sea level, but by reflecting solar radiation (up to 90%), the ice plays an important role in keeping polar temperatures low.
This is part of the explanation for the greater sensitivity of the polar regions to increasing greenhouse gas concentrations. Perhaps most worrying, the loss of the sea ice and consequent warming threatens to accelerate the so-called ‘slow feedbacks’ – the melting of large, land based ice expanses (eg. Greenland) and changes in the global carbon cycle (Arctic soils alone store somewhere between 500 and 1000 billion tonnes of carbon, much of which could be mobilised – and ultimately oxidized to CO2 – by permafrost melting and soil warming).
In both cases, the effects of these slow feedbacks are global, but because they remain relatively poorly understood (and — as the name suggests — are thought to respond extremely slowly), they’re excluded from the IPCC projections (which should – for this and other reasons — be considered conservative).
But some idea of their importance can be taken from evidence of the interaction between changing ice sheet expanse and greenhouse gas concentrations throughout past glacial cycles. In a recent paper, Hansen and colleagues estimated that these feedbacks may respond much more quickly than previously estimated, and that the equilibrium change in temperature expected for a given increase in GHG levels may be underestimated by half when these slow feedbacks are excluded. They conclude:
If humanity wishes to preserve a planet similar to that on which civilization developed and to which life… is adapted… CO2 will need to be reduced from its current 385ppm to at most 350ppm.
And indeed, there is much recent evidence pointing to a climate system responding more strongly in most cases than might be expected on the basis of IPCC projections:
- Greenland is experiencing large accelerations of outflow glaciers (possibly associated with the loss of buttressing ice at the coastal margins and meltwater penetrating to and lubricating the bases of glaciers) and satellite measurements of gravitational anomalies over the ice sheet point to corresponding and significant overall mass loss;
- Sea level is rising at an estimated rate of 3.3mm/yr, as opposed to the IPCC projected best estimate of around 2mm/yr;
- The CO2 sink capacity of the earth system (land and ocean sinks) appears to be in decline, with the result that a larger proportion of the CO2 that enters the atmosphere via anthropogenic sources remains there;
- The per-decade rate of change in temperature is accelerating – the rate over the instrumental temperature record is less than 0.05oC/decade, but in the last 25 years it has been close to 0.2oC/decade.
If that wasn’t bad enough, the growth of atmospheric CO2 is also being fuelled not only by a growing world economy, but by increasing carbon intensity (carbon output per unit GDP) in both developed and developing economies (which has been the case since the turn of the century, prior to which it was in decline). So much for decarbonisation of supply. As a result, CO2 concentrations since 2000 have increased at a rate greater than the most emissions-intensive of the scenarios that inform the IPCC modelling effort.
It’s grim, and admittedly, it would be much easier to just stick our fingers in our ears and refuse to hear that it’s happening. It will be interesting to see whether that’s the response of the federal government to the Garnaut Review, because its current target of a 60% reduction in emissions by 2050 (2000 baseline) is, in light of the above, a dangerous anachronism.