Sometimes I truly wonder if the so-called sceptics ever take the effort to do the bare minimum of research before they attack the IPCC. This time I wondered this thanks to the guest blog post “What would the IPCC have written if there had been 12 years of rapid warming?” (archived here) that Anthony Watts deemed worthy to be published on his blog.
In this particular blog post Stephane Rogeau proposes two situations. One situation where the IPCC readily admits that the rapid warming is in part due to natural variability. And one where the IPCC uses this as evidence for the dire impact we humans are having on the climate.
This is the text that Rogeau says could be written by the IPCC if they would honestly write about it in their report:
“The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 2000 to 2012). Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 12 years (2000–2012; 0.23 [+0.13 to +0.33] °C per decade), which begins after the effect of a strong El Niño disappeared, is bigger than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).
The observed extra increase in surface warming trend over the period 1998–2012, as compared to the period 1951–2012, is due in roughly equal measure to an increased trend in radiative forcing and a warming contribution from internal variability, which includes a possible redistribution of heat within the ocean.”
What you need to know is that I had to read big chunks of the 2007 report for my project ‘Climate Changes, But Facts Don’t: Debunking Monckton‘ and the example from Rogeau sounded awfully familiar. The reason is that there was a period of rapid warming at the end of the last century. Something the 2007 report commented on.
Take for example this graph that is in the 2007 IPCC report:
It shows changes in temperature trends over the past century and as you can see the trend is getting steeper. The interesting part crops up when they talk about the meaning of those trends, like they do on page 336. Mind you this is the same chapter where this graph is used for the first time (emphasis mine):
Another low-pass ﬁlter, widely used and easily understood, is to ﬁt a linear trend to the time series although there is generally no physical reason why trends should be linear, especially over long periods. The overall change in the time series is often inferred from the linear trend over the given time period, but can be quite misleading. Such measures are typically not stable and are sensitive to beginning and end points, so that adding or subtracting a few points can result in marked differences in the estimated trend.
This is the IPCC report telling you it is dangerous to draw any conclusions from linear trends. Especially when they are calculated over a short time frame. The same thing the proposed text from Rogeau states.
What matters for trends like these is the context in which the warming happened. So the IPCC would investigate how much natural variability played a role in the increase in temperature and which parts of our climate system are responsible.
And they mention one reason for this rapid warming in chapter 3 “Observations: Surface and Atmospheric Climate Change” where this graph first appears, it says on page 246:
…only in the last decade is an overall warming signal clearly emerging. Therefore, the recent strong warming appears to be related in part to the AMO [Atlantic Multidecadal Oscillation] in addition to a global warming signal.
There’s more in the 2007 report about natural variability and how this influenced the temperature trends over the past century. The reason I quoted these passages is that the graph I showed is misrepresented by Monckton and I had to research his claim. When Monckton talks about this graph he makes the claim that the “2007 UN report deploys a flagrantly fraudulent statistical technique to pretend that the world is warming ever faster and that we are to blame.” But the IPCC doesn’t do that, as shown by few passages that I already quoted from the report.
So when Rogeau says this:
Let’s be honest: does anybody believe the IPCC would have chosen to write anything close to the first analysis?
I can honestly answer: I do!
Because the IPCC already did that for periods where natural variability influenced our forcings on the climate system. This is something they do for all temperature trends as without knowing what role natural variability played you can’t say anything about our contribution to the temperature trend.