IPCC: Yes, humans are definitely behind all this global warming we aren't having

Comment The Intergovernmental Panel on Climate Change says it's more certain than ever that humanity is warming the planet dangerously - despite the fact that a long-running flat period in global temperatures is well into its second decade.

The IPCC released a brief "Summary for Policy Makers" today (go here) a teaser for its hefty summary of the scientific evidence for climate change, which is to follow in a few weeks.

As leaks had suggested, the IPCC has increased its "confidence" that the noticeable warming experienced in the last part of the 20th Century was predominantly man made - but sidesteps explanations of why it went away. CO₂ has continued to increase rapidly this century, topping 400ppm.

In fact, the Summary doesn't mention "pause" or "hiatus" once. Skeptics argue that the IPCC's increased confidence is hard to justify for two reasons: firstly the climate models failed to predict the long pause (and over-estimated warming by between 71 and 159 per cent, according to Bjorn Lomborg) and secondly the explanation of the pause lacks a solid empirical basis.

"Climate models have improved since the AR4," the IPCC insists nonetheless, in its new WG1 Summary.

The IPCC issues its reports every seven years, and this is the fifth batch since 1990. The IPCC's three working groups deal with, respectively, the scientific evidence (WG1), impacts (WG2) and policy responses (WG3), with each running to several hundred pages. WG2 and WG3 report next year and the full WG1 has yet to be released.

The IPCC is several things, but it isn't, as is widely supposed, predominantly a UN-funded organisation. There is a small IPCC secretariat in Geneva that deals with administrative issues such as travel expenses, but the main process is paid for by national governments, who also select the scientists who write the first drafts.

Nor is the IPCC process predominantly populated by scientists, at least not after the first preliminary and informal discussions. Participation in the process is also voluntary, and from the second stage of discussions on, national-level bureaucrats and activists become involved. All this may sound arcane, but it accounts for the flavour of the reports. The process culminates, in the months leading to publication, in an unwieldy travelling circus of some 18,000 delegates meeting to horse-trade points.

How does the IPCC arrive at its confidence number?

The SPM tells us that: "Probabilistic estimates of quantified measures of uncertainty in a finding are based on statistical analysis of observations or model results, or both, and expert judgment".

As climate scientist Professor Judith Curry of Georgia Tech helpfully explains:

"The 95% is basically expert judgment, it is a negotiated figure among the authors.  The increase from 90-95% means that they are more certain.  How they can justify this is beyond me."

At the IPCC press conference today, Professor Thomas Stocker, co-chair of WG1, told press that "we're confident because we're confident". ®