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IPCC Report on the Drivers of Climate Change

September 30, 2013; 3:23 PM ET

This is part II of the key points from the IPCC's 5th Assessment report (Working Group I) on climate change, which was released Friday morning......

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The natural and man-made drivers of climate change

Total radiative forcing (RF) is positive, which has led to an up take of energy by the climate system which leads to warming.

Radiative forcing (RF) is defined by the IPCC as the change in energy fluxes caused by changes in natural/man-made drivers for 2011 relative to 1750.

The IPCC image below shows the RF by emissions and drivers.

The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m/-2, and it has increased more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than that reported in the 4th Assessment Report (AR4) for the year 2005. This is caused by a combination of continued growth in most greenhouse gas concentrations and improved estimates of RF by aerosols indicating a weaker net cooling effect (negative RF).

The total natural RF from solar irradiance changes and stratospheric volcanic aerosols made only a small contribution to the net radiative forcing throughout the last century, except for brief periods after large volcanic eruptions.

Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

It is extremely likely that 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. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.

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. 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. Together these assessed contributions are consistent with the observed warming of approximately 0.6°C to 0.7°C over this period.

Climate models

The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012).

The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle.

There has been substantial progress in the assessment of extreme weather and climate events since AR4. Simulated global-mean trends in the frequency of extreme warm and cold days and nights over the second half of the 20th century are generally consistent with observations.

There is robust evidence that the downward trend in Arctic summer sea ice extent since 1979 is now reproduced by more models than at the time of the AR4, with about one-quarter of the models showing a trend as large as, or larger than, the trend in the observations. Most models simulate a small downward trend in Antarctic sea ice extent, albeit with large inter-model spread, in contrast to the small upward trend in observations.

Dr. Chris Forest, associate professor of climate dynamics at the Pennsylvania State University and a lead author on chapter 9, the Evaluation of Climate Models, of the IPCC report told AccuWeather.com that a key factor in understanding the way climate models work is to not think of them as predictions but rather as projections.

"Given a possible future scenario of factors that are affecting climate change, we can run a climate model to produce the response of the climate system to those scenarios," he said. "It's not going to be perfectly predicted, but we can see the path that we're on for the next multiple decades, so that's something that's quite straight forward."

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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