By Andy May
I just gave an informal Zoom talk to a small group on measuring climate change through temperatures. The host, Dave Siegel, recorded it and has posted the presentation here, if you want to view it. It is about 15 minutes, plus some discussion afterword.
The key points of the talk are:
- The IPCC and I agree that temperature is a key indicator of the changing state of the climate system.
- The IPCC has traditionally used the global mean surface temperature (GMST) to estimate global temperature change, it has an organized database behind it, but atmospheric temperatures are very chaotic, so it might not be meaningful from a climate perspective.
- The new GSAT (global surface air temperature) model-based measure of warming is highly problematic, if it is introduced, as planned, in AR6. It is calculated with a model from GMST and increases the warming rate by 4%. Models suggest that GSAT is warming faster than GMST, but the data that exists does not support this extra warming. The data we have are mainly night marine air temperature measurements from ships.
- The ocean mixed layer is in constant communication with the surface and has 27x the heat capacity of the entire atmosphere. It covers 71% of Earth’s surface and does not react to short-term chaotic swings in atmospheric temperature. As a result, it is a more stable long-term record of climate change.
- The deeper ocean, below the mixed layer, is a record of temperatures in the past.
- A model is needed to build a good temperature record from current deep ocean temperatures plus proxies from ocean floor sediments.
- The phrase “climate change” is redundant, climate always has always changed and always will, we should just say “climate.”
The final slide of the presentation illustrates what can be done, it uses data from Yair Rosenthal, 2013, Science.
The left-hand graph shows a temperature reconstruction by Yair Rosenthal and colleagues in their 2013 paper in Science. They use bottom-dwelling foraminifera in the Makassar Strait, between Sulawesi and Borneo in Indonesia. The water at about 500-meters, where the forams live, is sourced from the Southern Ocean near Antarctica, the southern Indian Ocean and the North Pacific. This location is ideal for checking the 500-meter water temperature for much of the Southern Hemisphere and a portion of the Northern Hemisphere.
Deeper water is more insulated from the surface and the trends reflect longer-term climatic changes, uncontaminated with atmospheric variability.
On the right we see a location map and a temperature profile for the Makassar Strait from the University of Hamburg database. The database is a high resolution (0.25° latitude and longitude) monthly series that uses all available data from many years. This profile gets most of its data from 2004-2016. It shows an average temperature, at 500 meters, of about 7.7°C. Thus, this area, at 500 m, warms about 0.5°C from the depths of the Little Ice Age. Here the low temperature was 7.2°C in 1810.
The Holocene Climatic Optimum is identified on the graph, and in this strait, the temperature was often over 10 degrees, the Medieval Warm Period was about 8.5°C, much warmer than today.
In summary, the data we need to reconstruct Holocene, and older temperatures are in the oceans and in ocean sediments. Ocean temperature reconstructions represent much more of Earth’s surface (defined as from the ocean floor to the top of the atmosphere) than any land- or ocean-based measurements in the atmosphere. The atmosphere is too chaotic and unstable to give us representative climatic trends. Ocean temperatures are more stable, usable, and easier to compare to paleo-temperatures.