Are CFCs responsible for global warming?

A recent report says CFCs, not carbon dioxide, are the culprit in climate change. weegeeboard/Flickr

On Monday, a report appeared in the Australian claiming a link between chlorofluorocarbons (CFC) in the atmosphere and global warming. The report claimed “banned aerosols” and not carbon dioxide were “responsible for global warming since the 1970s”. It went on to claim global temperatures would fall as the concentration of CFC in the atmosphere declined.

The story was based on a recent article from an academic at the University of Waterloo in Ontario. It says:

Most conventional theories expect that global temperatures will continue to increase as CO2 levels continue to rise, as they have done since 1850. What’s striking is that since 2002, global temperatures have actually declined – matching a decline in CFCs in the atmosphere … My calculations of CFC greenhouse effect show that there was global warming by about 0.6°C from 1950 to 2002, but the earth has actually cooled since 2002. The cooling trend is set to continue for the next 50-70 years as the amount of CFCs in the atmosphere continues to decline.

The premise is false.

Global temperatures have continued to rise even though concentration of CFCs in the atmosphere are not going up. In fact, despite the stabilisation of CFCs since 1990, troposphere temperatures have continued to grow by approximately 0.3°C.

The contention that Earth has cooled since 2002 has lots of problems. It depends on choice of indicator choice of data set and the use of extremely short periods to infer long-term global warming.

Warming and “heat accumulation” is measured through various different climate indicators. One of the most cited is global-mean surface temperature. The last decade has shown continued warming in global mean surface temperature, not cooling. The warming has occurred at a slower rate in the past decade compared to the 1990s, but it has still warmed. The years 2006 and 2010 were the warmest peaks since 1998.

However, this is balanced by the fact that ocean heat content (a measure of heat accumulated in the ocean depths) has dramatically increased over the same period at 0 to 700 meter depth. That draw-down of heat into the oceans has resulted in slower warming in the atmospheric temperature at the surface of the oceans, which transfers to slower warming in the global mean surface temperature.

It is also likely that aerosol emissions (mainly from China) since about 2002 have cooled the earth.

All the extra heat going into the oceans is not a good news story, and does not mean climate change has stopped or reversed. Land-surface temperatures alone have warmed strongly in the recent period. Warming over the last decade is also evidenced by increasing temperatures in the troposphere (the atmosphere above the surface), decreases in northern hemisphere snow cover extent, increases in global-mean sea level and decreases in Arctic sea-ice.

CFCs’ contribution to global warming has been small. The atmospheric energy rise is the first step to warming. The atmospheric energy rise of CFCs since 1750 has been about 0.34 Watt/m2. By comparison CO2 contributes 1.66 Watt/m2, methane 0.48 Watt/m2 and N2O ~0.16 Watt/m2.

The report is right that CFCs have declined. In about 1990, CFC-11 started to decline. Since about 1998 CFC-12 leveled. Yet temperatures continued to rise because of accelerated rise of CO2 (it rose at an unprecedented rate of 2.98ppm/year between March 2012 and March 2013) and continuing growth in methane and N2O (see Figure 1).

Figure 1: Global trends of the mixing ratios of the well-mixed greenhouse gases versus time from NOAA observations. http://www.esrl.noaa.gov/gmd/aggi/