By RP Siegel,

The folks at Skeptical Science have put together a review of various scientific investigations into the causes of global warming, in hopes of coming up with a definitive answer. This seems like a good time to do this, in the midst of Republican primary season, as the various candidates try to one-up each other on bashing the science in lieu of what their supporters would prefer to hear.

Eight different studies were reviewed, dating from 2000-2012, with the average being just over four years old.

The results are summarized in the chart shown below which shows the causes of global warming over the past 50-65 years, according to six of the studies reviewed that used a variety of methods to reach their conclusions. (The other two studies not shown: Stott[2010] and Foster & Rahmstorf [2011] found the human contribution to be 86% and 100% respectively). The human contributions are shown at the left and natural contributions are shown on the right. As you can see from the chart, in many cases the natural factors actually contribute to global cooling, which is why some of the bars on the left show human contributions to be more than 100%, as they more than offset the naturally occurring cooling trend.

This is a clear message that shows an overwhelming consensus that most if not all of the warming over the past 50 years has been as the result of human activity, despite the fact that each of these studies used different methods to arrive at these conclusions.

The studies all concentrated on the same main contributing causes including:

  • Human greenhouse gas emissions – gases released as the result of human activity which remain in the upper atmosphere and reflect heat back down to the Earth that would otherwise escape into space.
  • Solar activity – normal variations in the sun’s radiative output due to the Earth’s orbital position, sunspot activity, or other such causes.
  • Volcanic activity – relatively short term cooling effect of sulfate aerosols dispersed in the atmosphere that block sunlight and reduce the amount of solar radiation reaching the surface.  Periods of reduced volcanic activity could cause more perceived warming.
  • Human aerosol emissions – (primarily sulfur dioxide [SO2]) which also tend to cool the planet. However, aerosols have a number of different effects (including directly by blocking sunlight, and indirectly by seeding clouds, which both block sunlight and increase the greenhouse effect), the magnitude of their cooling effect is one of the biggest remaining uncertainties in climate science.
  • The El Niño Southern Oscillation (ENSO) – an oceanic cycle which alternates between El Niño and La Niña phases.  El Niño tends to shift heat from the oceans to the air, causing surface warming (but ocean cooling), whereas La Niña acts in the opposite manner.

A brief summary of the studies follows with a brief overview of the methodology, time period studied, what the background natural climate trend was over that period and the overall percentage contribution resulting from human activity..

Tett et al. (2000) used an “optical detection methodology” with global climate model simulations to try and match the observational data.

Period studied: 1897 to 1997.

Natural climate trend: Cooling

Computed human contribution: >100%

Meehl et al. 2004 used a similar approach to Tett et al., running global climate model simulations using different main factors.

Period studied: 1890 to 2000.

Natural climate trend: Warming until 1950, cooling since then.

Computed human contribution: 80% until 1950, >100% since then

Stone et al.used 62 climate model simulation runs that incorporated IPCC findings to run models up to date and project  forward to the year 2080 as part of a challenge. They found that close to half of the human contribution was offset as the result of aerosols which had a cooling effect.

Period studied: 1940 to 2005 and 1901 to 2005 (Two separate studies)

Natural climate trend: Warming until 1940, cooling since then.

Computed human contribution: 50% until 1940, close to 100% since then

Lean and Rind 2008 used a more statistical approach, incorporating measurements of solar, volcanic, and human influences, as well as ENSO, and statistically matched them to the observational temperature data to achieve the best fit.

Period studied: 1889 to 2006.

Natural climate trend: Warming.

Computed human contribution: 80%, close to 100% since 1955

Stott et al. used statistical regression results to constrain simulations from five different climate models and corroborated their results by looking not only at global, but also regional climate changes.

Period studied: 20th Century.

Natural climate trend: Warming.

Computed human contribution: 86%

Huber and Knutti 2011 utilized the principle of conservation of energy for the global energy budget to quantify the various contributions to observed global warming. They also estimate that more than 85% of the heat has been absorbed by the oceans.

Period studied: 1850 to 2000s.

Natural climate trend: Warming.

Computed human contribution: 75% since 1850, 100% since 1950.

Foster and Rahmstorf (2011; FR11) examined five different temperature data sets, including satellites

Period studied: 1979 to 2010.

Natural climate trend: Cooling.

Computed human contribution: >100%

Gillet et al. used a statistical multiple linear regression approach, applied to the second generation Canadian Earth System Model.  They used data for human greenhouse gas and aerosol emissions, land use changes, solar activity, ozone, and volcanic aerosol emissions.

Period studied: 1851 to 2010, 1951-2000, and 1961-2010

Natural climate trend: Cooling.

Computed human contribution: >100%

These combined results constitute definitive evidence of the impact human activity has had on this planet’s climate, particularly over the past 50-65 years. Of course there will still be skeptics, since so many people only hear what they want to hear. Evidence like this should perhaps give some of those people pause, while compelling the rest of us to take urgent action in every sphere of our lives.

[Image credit: Courtesy of Skeptical science]

RP Siegel, PE, is the President of Rain Mountain LLC. He is also the co-author of the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water. Now available on Kindle.

(Source: www.triplepundit.com )