The Physics of Climate Change by Lawrence M. Krauss

Author:Lawrence M. Krauss [Krauss, Lawrence M.]
Language: eng
Format: epub
ISBN: 9781800244795
Publisher: Head of Zeus

Figure 7.133

As can be seen, the actual change, accounting for the year-to-year fluctuations is in the range 1.15–1.35°C. That is pretty good agreement with the prediction.

In fact, the agreement is actually better than one might have expected because, as I indicated, the prediction I presented assumes just a change in CO2 abundance without considering other possible radiative forcing factors present, such as additional greenhouse gases, surface albedo, clouds, solar irradiance, or pollution. Some of these, such as increased cloud cover or conversion of forests into urban areas, produce a negative forcing, whereas others, including contributions from other greenhouse gases such as methane and other hydrocarbons, produce a positive forcing.

In 2007 the UN Intergovernmental Panel on Climate Change (IPCC) released a chart showing the best estimates and the predicted uncertainties for various radiative forcing factors for 2005 compared to 1750. The largest relative uncertainties involved the effect of clouds and aerosols in the atmosphere, while the predicted uncertainty associated with CO2 abundance was relatively small. When combined, the average predicted radiative forcing at that time was about 1.6 W/m2. This is about 5 percent smaller than that predicted for CO2 alone, confirming that the estimate we obtained by just considering CO2 was a reasonably good approximation to the full situation.

Alas, humanity has not stood still since 2005. Greenhouse gas emission has continued to rise. We remain on track to double the CO2 concentration since 1750 in this century. Figure 7.2 is a chart showing the forcing due to greenhouse gases since 1980.

At the same time, uncertainties in various other factors, including impact of aerosols and clouds, have decreased. In 2013, the IPCC updated their chart, also listing the increase in net anthropogenic radiative forcing over the years. Thus, the net forcing in 2011 was estimated to have a mean value of 2.29 W/m2, a 40 percent increase compared to the 2005 estimate, and almost double that estimated from 1980. The chart is shown in Figure 7.3.

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