A couple of years ago I wrote a joint post with Roger Pielke Sr that discussed assessing anthropogenic global warming. The post basically used changes in ocean heat content to assess anthropogenic global warming. The basic idea (which is not a new one) is that anthropogenic global warming is driven by our emission of greenhouse gases (mainly CO2) into the atmosphere, which acts to reduce the outgoing energy flux. This produces a planetary energy imbalance and means that energy will accrue in the climate system until it returns to energy balance via an increase in surface temperature.
One reason I thought I’d write about this is that I came across an article called taking the pulse of the planet, which John Abraham (one of the authors) also discusses in this Guardian article. This article also discusses using ocean heat content to assess anthropogenic global warming. A key point is that most of the excess energy associated with the planetary energy imbalance goes into the oceans, with only a small fraction heating the land/atmosphere. Therefore, surface temperatures show much more variability than ocean heat content measurements.
What this means, as the table on the right shows, is that it may take more than 20 years for a trend to emerge from the noise in surface temperature data, but it only takes about 4 years if you consider ocean heat content (OHC), or sea level rise (SLR), data. Therefore, ocean heat content (and SLR data) are more robust indicators of global warming, than surface temperature data.
The authors of the article therefore recommend that
that both the EEI and OHC be listed as output variables in the CMIP6 models, in addition to SLR and GMST.
where EEI is the Earth’s energy imbalance, and GMST is global mean surface temperature.
I think the above is quite an important suggestion, because I do think it’s important to stress that anthropogenic global warming (AGW) is really about energy accruing in the climate system due to us emitting greenhouse gases into the atmosphere; it’s not just about rising surface temperatures. The surface temperature does, of course, rise as a consequence of this increasing energy (and is the way in which the system returns to energy balance), but assessing AGW should probably be done by considering the overall energy balance, rather than by simply considering surface temperatures (from which it is much more difficult to extract a signal from the noise).
Of course, I’m not arguing that it’s not also important to consider surface temperatures; we do live on the surface and – of course – want to understand how surface temperatures are rising, and will continue to rise, in response to increasing anthropogenic forcings. However, by considering the overall change in energy we should get a better sense of the rate of AGW and might avoid claims – based on looking at noisy data over short time periods – that global warming has “paused”. Okay, the latter is probably naive, but I’m not completely without hope.