The 40-Year Climate Delay Between Cause and Effect

by Ray Grigg

One of the obstacles preventing people and governments from fully appreciating the consequences of their carbon dioxide emissions is a condition that climatologists call “climate lag”, the “thermal inertia” which delays by about 40 years the warming effect of rising levels of atmospheric greenhouse gases.

This means that the climate we are getting in 2015 was generated about 1975 when atmospheric carbon dioxide concentrations were 334 parts per million, and the future climate for our children in 2055 will be based upon the 400 ppm we have today.

This 40-year delay, calculated by one of the world’s foremost climate experts, James Hansen, is an approximation based on a range extending from 25 to 50 years. Since the median is 37.5, a simplified estimate is 40 years.

Climatologists know that the average global temperature has increased 0.8°C from all accumulated carbon dioxide emissions since 1800. Because we have yet to receive most of the effects of our emissions during the last 40 years, climate lag will eventually add another 0.6°C to this increase. Since global CO2 emissions are still going up rather than down (0.5% in 2014), we will very likely be unable to keep the temperature increase below the 2°C agreed to by the world community.

If the weather extremes we are getting now make us uneasy, the weather eventually generated from our current emissions will be considerably more disquieting.

Neither will we like the permanence of these new conditions. About one-third of all the carbon dioxide we emit is added to the atmosphere and will stay there for at least 1,000 years before being safely sequestered by natural processes. From the perspective of our civilization, the climate change we are causing is essentially permanent.

“Climate lag” requires an explanation of “thermal inertia”, a subject that seems complicated because we usually think of global warming in terms of air temperature. But most of the planet’s accumulating heat actually goes into the oceans. They cover about 70% of Earth and constitute 500 times the mass of the atmosphere.

The oceans, therefore, serve as vast heat sinks, masking our perception of the actually warming that is taking place. Because the oceans churn and circulate in ways we do not yet fully understand, the colder and warmer currents reaching the surface absorb atmospheric heat at different rates, causing apparent irregularities in our temperature readings. In reality, however, the entire mass of the planet’s surface is heating in direct relationship to the amount of carbon dioxide in the atmosphere — without any of this gas, global temperatures would be about 35°C colder, or about –21°C.

Put simply, a portion of every kilogram of carbon dioxide we have ever emitted from burning fossil fuels is translated into heat that eventually manifests as a more active climate. Warmer oceans are a key part of this process, inexorably causing more intense storms, more precipitation extremes and more unusual weather.

The good news for many of us alive today is that the full consequences of our present emissions will not be known for another 40 years. The bad news is that they will be our bequest to future generations.