Climate Change 101 - Part 1 revisited

I reread what I wrote the other day and wanted to say a little more about the contributions of natural forcing to climate change. I don't mean to say that the current changes are ALL due to humans. There is definitely input from natural sources. But the consensus among climate scientists is that these sources are far outweighed by human activities. The 169 authors of the latest IPCC (all of them distinguished scientists in their respective fields) agreed that there is a 90% chance that “the human influence on climate dominates over all other causes of change in global average surface temperature during the past half century.”

I welcome your comments. Let me know what you think! Thanks for reading ...

A large portion of the year-to-year variability in the earth’s climate is due to cycles in El Nino/La Nina. However, while this pattern can explain temperature and precipitation changes on short time scales, it does not account for the strong warming trend seen throughout most of the 20th century. For one thing, the pattern of 20th century warming includes stronger warming over lands than oceans and stronger warming in high latitudes. El Nino is a phenomenom that occurs in the tropical Pacific ocean. So the spatial pattern of El Nino does not match up with the 20th century warming.

There is an 11-year cycle in solar variability, and solar output has increased over the past couple of centuries. However, warming from an increase in solar radiation would be expected to be found in both the troposphere (the lowest part of the atmosphere) and higher up in the stratosphere. But 20th century warming has been greatest in the troposphere and the stratosphere has actually cooled. Increases in solar radiation have contributed somewhat to 20th century warming, but not as much as human activities.

Volcanoes can also contribute to climate change. On short time scales, volcanoes decrease global temperatures. Eras with heavy volcanic activity (such as the Cretaceous) were several degrees warmer than today, but there has not been a significant increase in volcanic eruptions during the 20th century.


Finally, we can use climate models to further assess the importance of human activities versus natural variability. The figure to the left (Figure 9.5 from the IPCC report) shows observed 20th century temperatures (black lines) and modeled temperatures (red and blue lines). In the top graph, the models (red) include both anthropogenic and natural climate forcing. The models agree well with the observed temperature trends, including decreased temperatures following major volcanic eruptions (such as El Chichon and Pinatubo) and a slight decrease in temperatures during the 1950’s and 1960’s. In the bottom graph, the models (blue) use only natural forcings (such as volcanoes, solar variability, and El Nino). These models do not include anthropogenic emissions of greenhouse gases and aerosols. In this case, the models do not show the increase in temperature that we have experienced over the past 50 years. This is further evidence that 20th century warming is mostly caused by human activities.

Source: IPCC FAQ: “Can the warming of the 20th century be explained by natural variability?”
You can download the complete answer to this and other good questions from http://ipcc-wg1.ucar.edu/wg1/FAQ/wg1_faqIndex.html.