What is the role of the oceans in Global Warming/Climate Change?

Global Conveyor Belt Ocean Currents

There has been much press and controversy in recent years about global warming or climate change.  Opinions range from doomsday scenarios to absolute disbelief.  As a meteorologist (not climatologist), I say that the truth is somewhere in between these extremes.

Although climatologists and meteorologists debate the merits of global climate change theory, there are some facts that are not disputed by scientists.  The vast majority of scientists agree that the global surface temperatures have increased since the mid-1800s and that humans are adding CO2 to the atmosphere. They also agree that CO2 and other greenhouse gases have a warming effect on the planet.  There is some disagreement, however, on the IPCC claim that there is a 95% certainty that the warming since 1950 has been primarily caused by human activities and just how much the planet will warm during the 21st century. They also tend to argue whether the warming is dangerous and whether or not anything can or should be done to prevent this.

Just what are greenhouse gases anyway?

Source: Smithsonian National Museum of Natural History http://forces.si.edu/atmosphere/images/02_04_06_b.jpg

Greenhouse gasses are those gases that allow the atmosphere to retain heat and thus warm the earth’s surface above what it would be from sunlight alone. The most significant greenhouse gas is water vapor followed by CO2, methane, nitrous oxide and ozone.  Sunlight heats the earth’s surface and that heat is then partially absorbed by greenhouse gases.  Without these greenhouse gases the earth’s temperature would be about 33°C (59ºF) colder than it is now, and our world would likely be a giant snow ball.

Atmospheric CO2 levels over time

The issue today is that some of these greenhouse gases, particularly carbon dioxide and methane have been increasing steadily since the beginning of the industrial revolution. Over the past approximately 150 years the level of carbon dioxide, for example, has undergone a very significant increase of about 40 percent, which cannot be accounted for by natural sources alone.  Carbon dioxide is a powerful greenhouse gas, however, it is present in our atmosphere in very small quantities, about 400 parts per million or 0.04%.

Estimates of CO2’s contribution to the greenhouse effect vary between 9% (3C) to as much as  26% (8.6C) of the total 33C effect.  Given that atmospheric CO2 has increased about 40% over the past 150 years then you might well expect some warming.  Just how much warming is what is in dispute and the best estimates I can find is that doubling CO2 would result in a warming of 0.7C to 1.2C alone or about 0.3C to 0.5C at the current 40% increase level.  Add in various feedbacks and you might double that number.

Natural Variability

The oldest instrumental temperature record comes from Central England and goes back to the 1600s.  In that record there is a sharp warming from 1690 to 1740 and another from 1820 to 1840.  Although we can’t infer too much from this about global temperature variations, the Central England temperature record does illustrate the magnitude of natural climate variability.

Source: Judith Curry Climate Etc. Blog http://judithcurry.com/2011/12/01/the-long-slow-thaw/

Solar activity also varies over time which affects global temperatures. From about 1750-1950 total solar irradiation has been estimated to have increased by about 1 to 1.5 Watts per square meter which can account for 0.2C to 0.3C warming during that 200 year period.  More recently, total solar irradiation has begun to slowly decrease.

Current Pause

Past 10 years NCEP CFSv2 2 Meter Temperature Anomaly C Source: weatherbell.com

Since 1998 there appears to be a pause in global warming not explained by the global climate models as they all predicted a 0.2C per decade warming.  Are the models too sensitive to CO2 or is the treatment of natural variability not modeled well?  Some papers suggest that the pause is not really there, however, the satellite data and the Nation Centers for Environmental Prediction (NCEP) 2 meter temperature data strongly suggest that it is.

IPCC Report

In the most recent (2013) IPCC report it states that “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased“.   

NOAA Global Land and Sea Temperatures

NOAA Reanalysis of SST Anomalies

This suggests to me that there is more in play here than just CO2, since the earlier warming occurred prior to when the majority of CO2 was emitted. Another theory offered by Dr. William Gray suggests that the ocean has multi-decadal and multi-century cycles that may have a significant influence on global warming. Below you see a similar trend with the ocean temperatures. 

The role of the Oceans

The total heat content of the oceans is enormous when compared to the atmosphere.  The atmosphere contains only about 2% of earth’s heat, the land masses another 2% and the oceans, about 93% with the remainder locked up in ice.  Given the above, one must ask just what role the oceans play in global warming?

The ocean-atmospheric interactions are not fully understood and given that the vast majority of earth’s heat is stored in the oceans it is vital to any predictions regarding global warming to understand those interactions.  One of the key questions is how much mixing of cold deep ocean water with warmer surface water varies over time and how does that effect the global temperature.

One key ocean cycle that is well documented is the Pacific Decadal Osculation (PDO).  The PDO is a warming and cooling of the Pacific Ocean over a time period of 2-3 decades.   If you plot the PDO index (see above) you will see that the two periods of strong warming (1910 – 1945) and (1978 – 1998) corresponds well with the positive (warm) PDO while the two periods of no global warming or some cooling corresponds well with the periods of negative PDO.

The theory that global warming is driven by ocean cycles seems to have some validity. Increasing atmospheric CO2 does have a role here, however, the longer-term ocean cycles may have an equal or possibly stronger effect. Besides the multi-decadal ocean cycles like the PDO, it is theorized that the Thermohaline Circulation (the global ocean current conveyor belt) runs in cycles that can extend out 100 years or more. As this circulation increases you get more mixing and thus colder sea surface temperatures in the tropical regions and warmer near the poles, however, if slows down you get less mixing and therefore warmer sea surface temperatures in the tropical regions but cooler SST in Polar regions.

Source:  The Physical Flaws of the Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver – Dr. William Gray   http://tropical.atmos.colostate.edu/Includes/Documents/Publications/gray2012.pdf

How could greenhouse gases warm the oceans?

The downward radiative forcing at the earth’s surface due to increasing CO2 levels is estimated to be about 0.4 W/m2. The oceans, on average, are warmer than the atmosphere so the net energy flux across the ocean/atmosphere interface carries heat from the ocean to the air. Given this and the fact that downward infrared energy cannot penetrate the surface of the ocean more than a few microns how then can back radiation warm the oceans?

The current theory suggests that the downward infrared radiation heats the very thin skin layer of ocean which has the effect of reducing the flow of heat from the ocean to the air, thus allowing the oceans to retain heat over time. Detractors of this theory point out that the increase in skin temperature would increase evaporation rates and thus tend to cool the skin surface largely nullifying any warming effect.

The spike in global temperature for 2014-2015 reflects, in part, a brief return to the positive PDO in late 2013 and as we return to a negative PDO in the coming years the current pause in warming will continue. By 2020  we may have a better idea on how much ocean cycles play in atmospheric warming.

See also Reasoning about climate uncertainty Judith Curry

Fred Pickhardt
Ocean Weather Services