Question

Discuss the difference between anthropogenic and natural causes attributed to climate change. Discuss whether or not you classify climate change as the major environmental health challenge of the 21st century. Explain your reasoning and provide evidence to support your claim.

Answer 1

There are other effects, but GHGs and SO2 are the two largest human influences, and solar and volcanic activity and ENSO are the dominant natural influences on global temperature. Now let's see these one by one.

natural influences on global temperature:-

Solar activity also warms or cools the planet by increasing or decreasing the amount of radiation reaching the Earth's atmosphere and surface.

Volcanic activity generally cools the planet over short timeframes by releasing sulfate aerosols into the atmosphere, which block sunlight and reduce the amout of solar radiation reaching the surface. However, unlike many greenhouse gases, aerosols are washed out of the atmosphere quickly, mostly after just 1-2 years. Thus the main volcanic impact on long-term temperature changes occur when there is an extended period of particularly high or low volcanic activity.

The El Niño Southern Oscillation (ENSO) is 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. As we'll see, a few studies have begun examining whether ENSO has had a long-term impact on global surface temperatures. Because it's a cycle/oscillation, it tends to have little impact on long-term temperature changes, with the effects of La Niña cancelling out those of El Niño.

human influences on global temperature:-

Human aerosol emissions (primarily sulfur dioxide [SO2]) also tend to cool the planet. The main difference is that unlike volcanoes, humans are constantly pumping large quantities of aerosols in the atmosphere by burning fossil fuels and biomatter. This allows human aerosol emissions to have a long-term impact on temperatures, as long as we keep burning these fuels. However, because 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.

human greenhouse gas (GHG) emissions warm the planet by increasing the abundance of greenhouse gases in the atmosphere, thus increasing the greenhouse effect.

Which one is more responsible for climate change:-

Tett et al. (2000)

Tett et al. (2000) used an "optimal detection methodology" with global climate model simulations to try and match the observational data. The inputs into the model included measurements of GHGs in the atmosphere, aerosols from volcanic eruptions, solar irradiance, human aerosol emissions, and atmospheric ozone changes (ozone is another greenhouse gas).

Tett et al. applied their model to global surface temperatures from 1897 to 1997. Their best estimate matched the overall global warming during this period very well; however, it underestimated the warming from 1897 to 1947, and overestimated the warming from 1947 to 1997. For this reason, during the most recent 50 year period in their study (shown in dark blue in Figure 1), the sum of their natural and human global warming contributions is larger than 100%, since their model shows more warming than observed over that period. Over both the 50 and 100 year timeframes, Tett et al. estimated that,

natural factors have had a slight net cooling effect, and thus human factors have caused more than 100% of the observed global warming.

Meehl et al. (2004)

Meehl et al. 2004 used a similar approach to Tett et al., running global climate model simulations using various combinations of the different main factors which influence global temperatures (GHGs, solar activity, volcanic aerosols, human aerosols, and ozone), and comparing the results to the temperature data from 1890 to 2000. They found that natural factors could account for most of the warming from 1910 to 1940, but simply could not account for the global warming we've experienced since the mid-20th Century.

Meehl et al. estimated that approximately 80% of the global warming from 1890 to 2000 was due to human effects. Over the most recent 50 years in their study (1950-2000), natural effects combined for a net cooling, and thus like Tett et al., Meehl et al. concluded that human caused more than 100% of the global warming over that period. Over the past 25 years, nearly 100% of the warming is due to humans, in their estimate.