Over the past month or so, there’s been quite a bit of controversy over a scientific measurement called equilibrium climate sensitivity (ECS). Among the media, confusion abounds. In a recent instance, The Economist, taking a number of scientific studies out of context, made the dubious claim that a slower pace of temperature increase during the first decade of the 20th century indicated a lower level of climate sensitivity. Other news outlets continue to remark on new climate sensitivity studies without appearing to understand what equilibrium climate sensitivity really means or, more importantly, understanding the inherent limitations of model-based ECS estimates.
Because there’s been such a high level of interest in and confusion over ECS recently, it’s worthwhile taking a closer look at this measurement’s scope and limitations. In an effort to clear up some of the confusion surrounding ECS, this article will attempt to answer these questions:
- How is ECS defined?
- How accurate is ECS?
- And, lastly, does a slower pace of warming over the first decade of the 21rst century mean climate sensitivity is less than previously expected?
Definition of Equilibrium Climate Sensitivity
In its broadest sense, ECS defines the long-term increase in surface air temperatures that results from a doubling of atmospheric carbon dioxide. The measure is important because it gives a broad indication of how much climate change and related harm to humans and environments results from a given amount of carbon dioxide being pumped into Earth’s atmosphere.
Under these basic principles, ECS provides a good guideline. However, ECS operates under a major handicap. The measurement does not include the effects of slow climate feedbacks like loss of ice sheets or albedo change to Earth’s surface.
How Accurate is Equilibrium Climate Sensitivity?
Because ECS leaves out slow feedbacks, it isn’t a very accurate measure of potential long-term warming. That said, in the early days of global warming modeling, ECS was seen as the most useful measure because models had difficulty handling complex physical forces that resulted from slow feedbacks. The result was that climate science came to rely on a less accurate measure because it was more expedient to use in modeling.
For these reasons, ECS was developed as a simpler way to model atmospheric temperature increase caused by carbon emissions over the long term. And because it was difficult to model slow feedbacks, they were not included in the measurement. So, though ECS is a useful measurement for the purposes of more easily modeling atmospheric temperature increases, ECS dramatically undershoots long-term global temperature increases.
The reason for this is that slow feedbacks such as albedo change and ice sheet melt have powerful impacts. We know this because ECS models tend to present about half the total sensitivity observed in the paleoclimate data for a doubling of CO2. This second and more accurate, but far more difficult to model, measure of climate sensitivity based on all global feedbacks acting in concert is called Earth System Sensitivity (ESS).
In total, the combined Earth System Sensitivity is far greater than the more model friendly Equilibrium Climate Sensitivity. By how much? Based on paleoclimate data, total long term ESS is probably about double that of current ECS estimates.
Unfortunately, these different definitions can be very confusing to the layman observer. As an example, the IPCC estimates ECS to be between 1.5 to 4.5 degrees Celsius for each doubling of CO2. From an average of these measures, the IPCC gets its estimate ECS of about 3 degrees Celsius. On the other hand, observations of past Earth climates show temperatures averaging at least 6 degrees Celsius warmer when CO2 levels were around 560 parts per million, or about double what the IPCC estimates for ECS.
Yet ECS is, most often, the official, published estimate for how much the Earth will warm. Yet, as shown above, given our current understanding of past climates, the ECS model estimates are short by half.
When looking at the stunning impact of CO2 on global temperatures in the paleoclimate data, one wonders why ECS is used, so often, without this broader qualification? Why, instead, don’t estimates of ECS provide a broader indication that end temperature increases are likely be double those seen in the climate models?
Lower Climate Sensitivity?
For the most part, scientists are trying to determine if slower atmospheric warming since 2000 indicates that climate sensitivity, in this case an already, short by half, equilibrium climate sensitivity, is less than previously expected.
For context, average atmospheric temperatures increased by about .2 degrees Celsius during the 1990s while atmospheric temperatures during the 2000s increased by about .1 degree Celsius. This apparent slowdown in atmospheric warming has caused some to question whether equilibrium climate sensitivity is less than previously expected.
One paper, published by Alexander Otto in Nature indicated a long-term equilibrium climate sensitivity of about 2 degrees Celsius based on new data from recent decades. This new model estimate is still in the range of 1.5-4.5 degrees Celsius provided by most model runs. Furthermore, the study found almost no significant changes to equilibrium climate sensitivity in the long-term trend.
Another Nature study, conducted by Roger Bodman of Victoria University, also found that model estimates for equilibrium climate sensitivity were not lower than previous estimates.
The conclusions to draw from this information are manifold.
The first is that model estimates for equilibrium climate sensitivity are not the best measure of total, long-term climate change. For that we should take a look at past Earth climates. From these measurements, we can find an Earth System Sensitivity of about 6 degrees Celsius for each doubling of CO2. This measure is consistent with Earth climates 50-65 million years ago when CO2 measured about 580 parts per million and temperatures were more than 6 degrees Celsius warmer.
The second conclusion is that long-term change will likely take many centuries to completely unfold. So long-term climate sensitivity measures like ECS or ESS are not good indicators for how fast Earth temperatures will increase within a given decade. Some decades may see relatively slow increases, some little or no increase, and some remarkably rapid increases. However, the overall trend will be for warming, and probably a more rapid pace of warming than that seen in the geological record due to the fact that the human CO2 forcing is currently so powerful.
Furthermore, the .2 degrees Celsius warming during the 1990s and the .1 degrees Celsius atmospheric warming during the 2000s are not entirely indicative of what the long-term trend will look like. In all likelihood, natural variability favored warming more during the 1990s and less so during the 2000s. This is hinted at in the El Nino/La Nina cycle with many powerful El Ninos (which tend to warm the atmosphere) evident during the 1990s while La Ninas (which tend to cool the atmosphere) were more prevalent during the late 2000s.
The third conclusion is that major Earth system feedbacks are beginning to kick in that are likely to make observations of the climates of past decades less relevant. Loss of albedo, ice sheet melt, ocean heat uptake, and environmental carbon release all have a roll to play in future atmospheric warming. Together with a continued and growing human CO2 forcing, these and other factors will determine the pace of warming over the next century. Under business as usual, worldwide CO2 levels hit somewhere between 800 and 1000 parts per million by the end of this century. Such a strong forcing is likely to have very powerful and damaging effects on Earth’s climate system. Even the models that do not take into account slow feedbacks are showing warming of 5-7 degrees Celsius by the end of this century if business as usual emissions continue. And for such a high degree of warming to take place even without the additional contribution from a number of slow feedbacks would be a terrible result.
Fourth, it should be noted that ‘slow feedbacks’ are already beginning to emerge at a more rapid pace than previously estimated. One example, loss of sea ice, is already reducing the northern polar region’s albedo. Another instance, methane hydrate and tundra methane release, are also already adding a positive amplifying feedback to human-caused warming.
Finally, it is worth noting that a more rapid than expected melt of polar ice would temporarily keep temperatures lower at the cost of a more rapid pace of sea level rise combined with much more extreme weather. Such higher than expected paces of melt are entirely possible and, in certain regions, appear to be happening now. In one example northern polar sea ice has experienced an 80% loss of volume since 1980. The result is that northern hemisphere sea ice melt is occurring 80 years ahead of model projections.