Sunday, April 08, 2007

What is climate change?

[I reposted this to correct a number of formatting issues.]

The latest report from the Intergovernmental Panel on Climate Change (IPCC), the Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report, Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability, Summary for Policymakers, has this curious note near the end of the report:

Climate change in IPCC usage refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the Framework Convention on Climate Change, where climate change refers to a change of climate that is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.

That is a profound change and distinction. The change actually occurred in the Working Group I report back in February, but nobody noticed or noted it.

For reference, here is the definition of climate change from the Framework Convention on Climate Change:

"Climate change" means a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.

The fact that the older definition refers to "attributed directly or indirectly to human activity", whereas the newer definition refers to "whether due to natural variability or as a result of human activity" is rather significant.

The effect of this change is that the overall thrust of the IPCC reports rests less on a finding of effects from strictly human activity than the old framework convention. The effects of human activity are still there in the new reports, but categorized as "anthropogenic warming."

Even the February science report from IPCC noted a somewhat weaker overall linkage between warming and human activity than for the overall warming effect.

The language used in the latest report is that "it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems." That falls far short of attributing all or even the dominant share of the blame to human activity. The strongest statement the report makes is that "most of the observed increase in the globally averaged temperature since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations", but the report offers no definition for "most", leaving it to the reader to guess that it may mean simply greater than 50% of the observed increase. In short, even these reports don't suggest that global warming would be nonexistent if all human activity were removed from the equation.

In fact, the latest report explicitly states that "Limitations and gaps prevent more complete attribution of the causes of observed system responses to anthropogenic warming."

In short, the scientists are claiming greater than 66% certainty that human activity can be blamed for the effects and 90% certainty that human activity can be blamed for global warming, but they actually aren't offering a number for what fraction of global warming is due to human activity, telling us only that the effect is "discernible."

To me, the overwhelming conclusion should be that we need a lot more research and a much better understanding of the current climate and climate processes before we make too many bold pronouncements about what effects human activity has in fact had or may have had, let alone forecast what effects are likely to be seen in the future.

The report offers these thresholds for characterizing likelihood of an outcome:

  • Virtually certain - Greater than 99% probability of occurrence
  • Extremely likely - Greater than 95%
  • Very likely - Greater than 90%
  • Likely - Greater than 66%
  • More likely than not - Greater than 50%
  • Very unlikely - Less than 10%
  • Extremely unlikely - Less than 5%

And it offers these terms for expressing confidence in a statement:

  • Very high confidence - At least a 9 out of 10 chance of being correct
  • High confidence - About an 8 out of 10 chance
  • Medium confidence - About a 5 out of 10 chance
  • Low confidence - About a 2 out of 10 chance
  • Very low confidence - Less than a 1 out of 10 chance

-- Jack Krupansky

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