By Andy May
A lot of new information about sea temperature has been collected since I last wrote on the topic in 2016. In my last post on GHCN and the National Temperature Index, it was shown that the development temperatures of the oceans and the distribution of thermal energy in the oceans dominate climate change. Land-based weather stations are invaluable for weather forecasting, but they say little about climate change. The usual definition of climate is a general change in temperature or precipitation over a period of more than 30 years. But even 30 years is a short period of time, 100 years could be better. On this timescale, the trends in sea temperature are more significant.
Oceans cover 71% of the earth and contain 99.93% of the thermal energy ("heat") on the surface. Here we define the earth's surface as anything between the ocean floor and the top of the atmosphere, ~ 22 km. This calculation and the required references are shown in this table. As an example of the enormous impact of the oceans, we should consider that Earth's oceans contain more thermal energy than on the surface and in the atmosphere of Venus, where the temperature is 464 ° C or 867 ° F. In fact, the Earth's oceans contain four times more heat energy than the atmosphere of Venus, but the oceans have an average temperature of less than 5 ° C. A table with this calculation and the required references can be downloaded here.
We still don't have accurate information about the entire ocean, but we have a lot more than 2016. CSIRO has a nice data set here with temperature data from 2009 at 5,500 meters (Ridgeway, Dunn & Wilken, 2002). The University of Hamburg has several years of data here up to 6500 meters, but I couldn't read their NetCDF files with R. I tried both R NetCDF packages (ncdf4 and RNetCDF) and couldn't open their files either. If anyone knows how to read these files please let me know. In the meantime, the CSIRO NetCDF files have been able to be opened without any problems and we can work with their data even though it is only a year long. Figure 1 shows the average global CSIRO sea temperature from the surface up to 5,500 meters.
Figure 1. Global mean temperature from CSIRO 2009 from the surface up to 5,500 meters. Data source: CSIRO.
The temperature drops to a minimum of 1 ° C at ~ 4,250 meters and then begins to rise. The temperature distribution at 4,500 meters is shown in Figure 2.
Figure 2. CSIRO sea temperature at 4,500 meters. The white areas on the map are shallower than 4,500 meters.
At these temperatures, signs of thermohaline circulation can be seen. It is still unclear how often the sea water completely overturns. By overturning, we mean the time it takes for surface water to sink, to form a fully deep water cycle, and then to return to the surface. This process will likely take at least 1,000 years. It is the most important long-term heat exchange process on the earth's surface. So if the earth gets more heat energy from the sun, or CO2 or whatever causes warming, it will take a thousand years or more to circulate through the oceans. Figure 3 is a map showing the paths water takes through the deep ocean.
Figure 3. The main routes deep water takes as it moves from the surface into the deep ocean and then emerges a thousand or more years later. Notice that Antarctica is in the center of the map. This is because all oceans only meet in the Southern Ocean that surrounds Antarctica. Source: From Avsa – Wikimedia, CC BY-SA 3.0.
As we can see in Figure 3, surface water in the North Atlantic and the Southern Ocean is immersed in the deep ocean. It then begins to travel around the world, through all the oceans. It swells mainly in the Indian Ocean, the Southern Ocean and the Pacific. Since deep water emerges from the Atlantic, but mostly rises in the other oceans, the Atlantic has a slightly lower sea level than the other oceans. See also (Reid, 1961). Ascent from deep water is more common than descent. This NASA website has a good discussion and animation of thermohaline circulation.
Figure 2 shows some mixed temperatures in the South Atlantic alongside South America and southern Africa. This suggests that there may be some upward movement there. However, the greatest increases appear to be in the Pacific, the South and the Indian Ocean.
Discussion and mistake
Unfortunately, good data on sea temperature only go back to 2004. The data available to us suggest that the oceans are warming at a rate of 0.4 ° C per century. However, the ocean cycle time is more than 1,000 years and the record is only 15 years. So this is very speculative. However, if the oceans are really warming at a rate of 0.4 ° C per century, it is very unlikely that speculations about rapid and dangerous warming of the atmosphere would be cause for concern.
The Jamstec grid (Hosoda, Ohira & Nakamura, 2008) that we used for the shallower part (<2,000 m) of our analysis gives us an estimate of the error. It is an estimate of the spatial error, which can also be referred to as a lattice error. In other words, do we have enough data to make the map accurate? Figure 4 shows a map of this error by year and depth.
Figure 4. Jamstec lattice defects in degrees C. Data source: Jamstec.
As we can see in Figure 4, in 2001 the error was quite high until it reached a depth of around 1400 meters. Until 2004, depths below 1,000 meters were okay. As Figure 1 shows, depths of less than 1,000 meters vary widely and high errors are expected. These shallower waters interact with surface weather, especially in what is known as the "mixed layer". The mixed layer is a flat zone where turbulence has caused an almost constant temperature from top to bottom. The thickness of the mixed layer varies depending on the season and area, but averages about 60 meters. The mixed layer temperature reflects the surface temperature of the last few weeks in a complex way.
In our view, attempts to deduce the extent and rate of warming of the atmosphere only from measurements of the sea surface and atmospheric temperature are foolish and doomed to failure. The real "control button" for long-term temperature changes are the oceans. They regulate the surface temperatures through their enormous heat capacity. The mixed layer alone has 22 times the heat capacity of the entire atmosphere over 22 km. The regulation of atmospheric temperatures by the oceans also gives us plenty of time to determine if global warming is really a threat. We currently only have about fifteen years of ocean temperature data, but in another fifteen years we will have data for a "climatic" period. If the ocean warming trend is still less than one degree per century in 2035, we hardly need to worry.
I used R to do the calculations shown in the figures, but Excel to make the charts. If you want to check the details of my calculations, you can download my CSIRO R source code here.
None of that is in my new book Politics and Climate Change: A Story, But Buy It Anyway.
You can download the bibliography here.