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Climate Change

Degrees of separation: How warm do we think Earth is going to get, really?

Original Article

Proistosescu, C., and Huybers, P.J. (2017). Slow climate mode reconciles historical and model-based estimates of climate sensitivity. Science Advances 3, e1602821. DOI: 10.1126/sciadv.1602821

2-4.5 degrees Celsius (35-40 degrees Fahrenheit). This is the standard temperature range that has become synonymous with the way we conceive the consequences of climate change brought on by global warming. But recently, the Intergovernmental Panel on Climate Change (IPCC) Assessment Report updated the range to 1.5-4 degrees Celsius. Their reason: the panel could not offer a best estimate for how much the Earth was going to warm if carbon dioxide levels double (Fig 1). The cause harks back to a debate between climate researchers, where questions regarding these estimates were raised, and the community was left scratching their collective head. A new paper in Science Advances might have laid the debate to rest.


Fig 1. Simulations by the IPCC estimating the 2-6 degrees celsius rise in Earth’s temperature, driven by estimates of CO2 emission growth. (Source: ©2007 IPCC WG1 AR-4.)

Background bites

Believe it or not, but there have historically been two major temperature estimates of Earth’s warming. The first estimate is model based, with a range from 2-4.5 degrees, and scientists settled on this range based on two types of models: global climate models (GCMs) and paleoclimate records. GCMs use our acquired knowledge of the general circulation of Earth’s atmosphere and oceans, and utilize mathematical equations to simulate, understand and forecast climate change (Fig 2). Your local weather forecast uses this model!

Paleoclimate refers to Earth’s climate before humans used instruments for record keeping. By using environmental indicators such as tree rings, corals, ice cores and lake and ocean sediments, researchers can reconstruct our climatic history extending back hundreds or thousands of years.

The second estimate is lower: 1.5-3 degrees, and is based on historical instrumental records. These are human-recorded instrumental temperature measurements of air and ocean surfaces. The Central England temperature series is the longest running temperature record, dating back to 1659!

Fig 2. Climate models use the basic laws of physics, chemistry and fluid motion to run models and predict Earth’s response to changing climatic conditions. (Source: Wikimedia Commons)

The debate: which of these ranges is accurate?

Has the issue been resolved? If so, how?

It has! Cristian Proistosescu and Peter Huybers from Harvard University developed a mathematical model to parse out what made the estimate types (model based or historical records) different. But before revealing their conclusion, let’s take a quick look at an important climate fact: different parts of the planet warm at different rates, referred to as fast and slow modes of warming. The researchers state that fast warming usually occurs on land, primarily in the Northern Hemisphere. This means that when carbon dioxide levels increase in the atmosphere, the land quickly warms up. On the other hand, warming of oceans can take centuries to realize. Two main oceanic bodies, the Southern Ocean and the Eastern Equatorial Pacific, are the main contributors to this long-term warming effect. These fast and slow warming modes make the crux of the model generated by the researchers. Climate records based on historical records do not include the effect of the Southern Ocean and Eastern Equatorial Pacific to extrapolate their temperature estimates. This, according to the paper, explains the distinction between model and historical records based estimates. When both fast and slow warming patterns are introduced, the researchers found that temperatures fall within the expected range of 1.5 to 4.5 degrees for both models, and could potentially be higher, up to 6 degrees.

What is at stake?

A lot! Since even a 0.5 degree increase in temperature can cause vast devastation due to intensified storms and sea level rise. Having more accurate predictions regarding climate change is critical for us to prepare, evaluate and change laws and actions accordingly. In a world ridden with debates and controversies, it is valuable to have some consensus!



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