It is exciting to watch our understanding of climate merge in the scientific literature. Right now, many studies are coming out that investigate the slowdown of global temperatures; of problems accounting for all the energy in the Earth's climate system; and the role of natural and human factors in these questions.
First, let's establish a few facts. Increases or decreases in temperatures, particularly atmospheric temperatures, are not equivalent to increases or decreases in Earth energy – that is, the Earth can continue to warm whether atmospheric temperatures increase. Part of the reason for this is that most of the Earth's extra energy is stored in the oceans which have continued to heat over the past few decades with no cessation or pause.
With respect to the so-called slowdown, we've seen studies that show part of the "slowdown" in global atmospheric temperatures is associated with measurement problems. That is, are we measuring the entire globe, in particular the polar regions. Other studies point to internal movements of energy between various parts of the climate as an explanation for recent slowdown in atmospheric temperatures. In fact, this topic was discussed recently by my colleague Dana Nuccitelli.
When you think about it, how would you measure the entire globe temperature? You need to be certain you have enough measurements taken for long enough periods of time to be sure you can identify temperature trends. You have to be mindful of putting sensors in regions where there are artificial impacts to the temperature (such as in cities). You have to measure not only at the Earth surface but throughout the ocean depth and atmosphere height. How do you handle changes in instrumentation that occur over the many decades? How do you handle biases in temperature sensors? How do you position sufficient sensors across the globe and maintain them over time? These questions reflect the Herculean task facing climate scientists.
But perhaps the so called slowdown isn't just due to measurement problems? Perhaps something is blocking some of the sunlight from reaching Earth in the first place? We call such energy blocking a "negative forcing". If negative forcings are happening, we would expect to see an impact on global temperatures. I recently discussed a study which looks at combinations of measurement and negative forcings. Now I want to shift to a very new study that focused extensively on volcanoes.
In the study, Dr. Ben Santer and colleagues asked whether small volcanoes could be causing a slight reduction in the amount of sunlight that reaches the Earth. We've known for years that large eruptions can cause a temporary decrease in the Earth temperature but what about small eruptions? The authors found that in the past decade or so, the impacts of volcanoes are found in the optical depth (clarity) of the atmosphere and in temperatures of the lower part of the atmosphere (the troposphere). The authors input the results into computer models and compared the calculations to models that ignored volcanic effects. They discovered that when volcanoes are included, the models were more able to match the observed temperatures.
So, what does this all mean? First, it confirms an emerging picture amongst scientists. Is there a slowdown in warming or not? If there is, what is the cause? Volcanoes? Solar activity? Internal variability? The emerging consensus is the so-called slowdown primarily exists in the atmosphere (but not in the oceans) and it is being caused by all three of these factors. In a nutshell, global warming is continuing, despite what people want you to believe, and natural factors influence short term temperatures measurements.
It also means that the models are doing things correctly. When inputs to the model are improved, the fit to actual observations improve. Consequently, people who have incorrectly stated that climate models are too sensitive to the increased greenhouse effect (for example, John Christy in Senate testimony accessed here) need to rethink their positions. The models work, period.
The significance of the recent article is best captured by co-author Carl Mears who said,
Posted by John Abraham on Tuesday, 25 February, 2014
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