Our fingerprint on global warming
A question at the heart of the global climate change debate is: how do we know that our recent human activity is the cause of the current spike in global temperatures? The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that it is a greater than a 90 percent certainty that emissions of heat-trapping gases from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.” The evidence to support this assertion is strong.
It is true that the Earth’s temperature has varied in cycles as far back as the geological records exist, with periods of cooling and warming. It is also true that single one-off events can cause sudden inflection points in the climate pattern, such as a volcanic eruption or a change in the amount of radiation emitted by the Sun. However, the recent phase of global warming can be shown to be a result of our recent human industry and the science is straightforward.
Here are the three “fingerprints” which point to human activity as the cause of global warming.
1. The Kind of Carbon Found: The kind of carbon (or isotope) in the atmosphere that we observe today cannot come from natural sources in the quantities at which they are being observed. When we burn fossil fuel, the carbon that was trapped in the oil, natural gas or coal is released in the form of carbon dioxide. Carbon dioxide is a “greenhouse” gas — it traps heat. We know that the carbon dioxide in the atmosphere has come from burning fossil fuels because fossilised fuel sources are predominantly made up of the the lighter kind (isotope) of carbon (C12). Fossilised fuel sources come from decomposed plants and animals in the ground and these organisms when they were alive can only easily consume the lighter kind of carbon. They cannot metabolise the heavier isotopes of carbon (C13, C14). With the ratio of C12, C13 and C14 isotopes of carbon in the atmosphere recorded in numerous geological records and relatively constant in proportion over time, it is really easy to see when this ratio was imbalanced. We observe significantly higher levels of C12 in the atmosphere since the dawn of industrialisation, than over the previous 800,000 years of geological records.
2. Computer Models Match Observations: Accuracy of computer models for global temperature have been developed and tested and are consistent across scenarios with and without human variables. For a computer model to be considered accurate it must be able to reproduce the observed temperature patterns in the global environment. To date, when only natural factors — like volcanic eruptions and solar radiation intensity — are accounted for, the models stop accurately predicting global temperatures at the precise moment human industrial activity commenced. When human factors are added to the computer models, then they accurately predict the temperature patterns since industrialisation began.
3. Thickness of Atmospheric Layers: The depth of the troposphere relative to the stratosphere has increased in recent decades. The distance to the higher and cooler stratosphere has increased in recent decades. The troposphere contains the heat-trapping carbon dioxide. As the troposphere heats up, it expands in volume. We know it is not solar radiation causing the heating, which has happened at different times in the Earth’s past. If it was solar radiation then the stratosphere would also be heating up, but the upper layers of the atmosphere are not heating up in the same way. There have been times when solar radiation has caused temperature fluctuations for example the “Little Ice Age” between 1650 and 1850 has been shown to have been caused by a decrease in solar radiation. Solar radiation would have to pass through those layers before it could get to the troposphere, heating the higher layers as it did, if it were the cause of the heating. Since 1750, the average amount of solar radiation reaching the Earth has either remained constant or increased slightly. Scientists have in fact observed a cooling of the upper atmosphere.
1. How Do We Know that Humans Are the Major Cause of Global Warming (available athttp://www.ucsusa.org/global_warming/science_and_impacts/science/human-contribution-to-gw-faq.html#.VoUQz5N95Hc Accessed: Thursday, December 31 2015, 4:02 PM)
2. Climate change: How do we know? (available at: http://climate.nasa.gov/evidence/ Accessed: Thursday, December 31 2015, 4:02 PM)