IPCC AR5 Drops Tomorrow. Bone Up Today.
September 26, 2013
In anticipation of the new Fifth Assessment of the IPCC (AR5) which will be released tomorrow, there are a number of good backgrounders online. Here are some of the best.
Let me extract the key points and figures. Back in July, scientist Dana Nuccitelli summarized a new study, “Distinctive climate signals in reanalysis of global ocean heat content“:
- Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years. This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.
- As suspected, much of the ‘missing heat’ Kevin Trenberth previously talked about has been found in the deep oceans. Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.
- Some recent studies have concluded based on the slowed global surface warming over the past decade that the sensitivity of the climate to the increased greenhouse effect is somewhat lower than the IPCC best estimate. Those studies are fundamentally flawed because they do not account for the warming of the deep oceans.
- The slowed surface air warming over the past decade has lulled many people into a false and unwarranted sense of security.
And let’s not forget another key indicator of accelerating warming — the accelerating melting of the great ice sheets as documented in the most comprehensive analysis of satellite altimetry, interferometry, and gravimetry data sets to date:
Okay, so it’s clearly misleading to say the planet has stopped warming. What’s actually going on? It’s pretty nuanced: According to the leaked IPCC draft report, the rate of warming at the planet’s surface (technically, the “global mean surface temperature”) is lower over the last 15 years, kind of like a car easing off the accelerator. The draft states that the rate of surface warming from 1998-2012 was 0.05 degrees Celsius per decade. But over the entire period from 1951 to 2012, it was 0.12 degrees Celsius per decade. (Keep in mind that not every aspect of the climate system necessarily reflects this “slowdown”: Arctic sea ice, for instance, hit a record low in 2007 and then another record low in 2012.)
How significant is the surface temperature slowdown in the context of global warming as a whole? The slowdown is certainly big enough to measure—or else we wouldn’t be discussing it—but not a huge deal in the context of the climate system. That’s because surface temperature itself, while a useful measurement, only captures a small part of what’s actually happening to the planet.
Since 1975, global average surface air temperature has increased at a rate of 0.17 deg.C/decade (estimated by linear regression using either the NASA GISS or HadCRUT4 data sets). But the rate of increase hasn’t been perfectly constant over that entire time span.
As a matter of fact, there’s a 15-year time span during which the rate is notably different. Fifteen whole years!!! By at least one calculation, the difference is “statistically significant.”
Does this mean that global warming is wrong? That the computer models are utter junk? That this whole climate science thing is just a hoax, a nefarious scheme to cheat us all out of tax dollars in order to support the lifestyle of gaudy luxury that we all know scientists wallow in? (Science: money for nothin’ and your chicks for free…)
That 15-year time span covers the years 1992 through 2006, during which the rate of warming was 0.28 deg.C/decade. That’s a lot faster than the warming rate from 1975 to now.
Zeke Hausfather at the Yale Forum:
In recent years, a global network of automated buoys has offered a much-improved picture of what is going on below the surface in the ocean. These buoys automatically dive deep down into the ocean every day, taking temperature measurements as they slowly rise, and transmitting that data back to a central database via satellite. The figure below, via Argo, shows the location of buoys currently active in the world’s oceans.
While measurements of deep-ocean temperatures existed further back in the past, they were taken only in limited locations until 1999, when Argo buoys were widely deployed. However, scientists for far longer have been able to use more limited data to reconstruct temperatures down to depths of 2,000 meters, as shown in the figure below.
Total ocean heat content has increased by around 170 Zettajoules since 1970, and about 255 Zettajoulessince 1955. This increased temperature has caused the oceans (0-2,000 meters) to warm about 0.09 C over this period. As the UK’s Met Office points out, if the same amount of energy had gone into the lower atmosphere it would of caused about 36 C (nearly 65 degrees F) warming! The oceans are by far the largest heat sink for the Earth, absorbing the vast majority of extra heat trapped in the system by increasing concentrations of greenhouse gases.
And, of course, my latest video which should be sufficient to put down most denialist water-cooler climate experts in just a few minutes.