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Global warming: why is IPCC report so certain about the influence of humans?

 
Carbon Emissions

The IPCC is 95 percent confident that humans are the main cause of the current global warming. Photograph: Jianan Yu/Reuters

 

Powered by Guardian.co.ukThis article titled “Global warming: why is IPCC report so certain about the influence of humans?” was written by Dana Nuccitelli, for theguardian.com on Friday 27th September 2013 04.06 UTC

The fifth Intergovernmental Panel on Climate Change (IPCC) report states with 95 percent confidence that humans are the main cause of the current global warming. Many media outlets have reported that this is an increase from the 90 percent certainty in the fourth IPCC report, but actually the change is much more significant than that. In fact, if you look closely, the IPCC says that humans have most likely caused all of the global warming over the past 60 years.

Spot the Differences

Here is the relevant statement from the fourth IPCC report in 2007:

“Most of the observed increase in global average temperatures since the mid-20th century is very likely [90 percent confidence] due to the observed increase in anthropogenic greenhouse gas concentrations.”

Now here is the statement from the fifth IPCC report:

“It is extremely likely [95 percent confidence] more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.”

Did you spot the differences? The 2007 IPCC statement focused on human greenhouse gas emissions, while the 2013 statement pertains to all human influences on the climate. This includes the cooling effect from human aerosol emissions (pollutants that scatter sunlight).

Cooling from human aerosol emissions offsets about one-third of the warming from human greenhouse gas emissions. The new IPCC statement says that even taking that aerosol cooling effect into account, humans are still the main cause of the global warming over the past 60 years.

Current Global Warming Caused by Greenhouse Gases, Not Nature

The IPCC elaborates further. What’s causing global warming: human greenhouse gas emissions.

“The best estimate of the human-induced contribution to warming is similar to the observed warming over this period … The observed warming since 1951 can be attributed to the different natural and anthropogenic drivers and their contributions can now be quantified. Greenhouse gases contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3 °C over the period 1951−2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of −0.6°C to 0.1°C.”

What’s not causing global warming: natural external factors like solar activity, and natural internal factors like ocean cycles.

“The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from internal variability is likely to be in the range of −0.1°C to 0.1°C.”

We’ve observed about 0.6°C average global surface warming over the past 60 years. During that time, the IPCC best estimate is that greenhouse gases have caused about 0.9°C warming, which was partially offset by about 0.3°C cooling from human aerosol emissions. During that time, natural external factors had no net influence on global temperatures. For example, solar activity has been flat since 1950.

Solar Activity Vs Temperature

Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance (TSI; thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Krivova et al (2007). TSI from 1979 to 2009 from PMOD.

 

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