Climate Sensitivity, Part 1

A core goal in climate research today is the determination of climate sensitivity. That is, how sensitive is Earth’s climate to changes in energy inputs and outputs? Is the planet’s equilibrium temperature alarmingly sensitive or relatively insensitive to the additions of greenhouse gases in particular and other warming factors in general? What controls Earth’s climate sensitivity? Might these factors suggest something about the design of the Earth?

Several articles in the popular news hit last week in anticipation of the September 27 release of the Fifth Assessment Report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC). There’s little doubt that these reports will continue to reinvigorate the discussion about climate change. In that light, this article will discuss scientists’ efforts to determine the global climate’s sensitivity to changes in energy inputs and outputs.

Equilibrium climate sensitivity (ECS) is here defined as the response in global-mean (average) near-surface temperature to a doubling of atmospheric carbon dioxide (CO₂) from preindustrial levels (280 parts per million [ppm] to 560 ppm).¹ Although the average CO₂ level has not yet doubled from preindustrial values, ECS helps scientists estimate expected warming into the future. Some researchers use a similar measure known as transient climate response (TSR) to measure global sensitivity with respect to changes in radiative forcing (in this case, defined as the excess energy received by the Earth’s climate system) caused by greenhouse gases and other factors. TSR may be defined as the climate response after 70 years of 1 percent annual rises in CO₂concentration.²

Climate Sensitivity Uncertainty

Many factors, both manmade and natural, complicate the measurement of climate sensitivity. An accurate value involves many climate feedbacks, both positive (warming) and negative (cooling). In addition, variations in these factors imply that climate sensitivity itself may not be constant over time.

Global temperature depends on differences between the incoming and outgoing energy balance (known as radiative forcing). Climate sensitivity is a measure of the strength of this dependency.³ However, this relationship is highly complex and not entirely understood. In particular, changes in ocean circulation and accompanying modifications to regional atmospheric circulations complicate climate sensitivity analyses. The uncertainty of various feedback factors—such as regional atmospheric circulation, clouds, water vapor, land cover, and others—probably explains why little change in the range of climate sensitivity with respect to global climate models (GCMs) has occurred since the 1979 Charney Report.⁴ That research established an average of 3°C for the value of climate sensitivity. Scientists who monitor such factors as (1) the climate of the past 1,000 years, (2) climatic response to volcanic eruptions, (3) global temperature change since the ice age, and (4) geological relationships between CO₂ and climate, have attempted to constrain the value of climate sensitivity. Such unresolved internally variable climate feedbacks appear to play a key role in climate sensitivity uncertainty.⁵

The Range of Earth’s Climate Sensitivity

Scientists have estimated that actual global average surface temperatures have risen by 0.8°C during the last 125 years.⁶ Climatologist Michael J. Ring and his research team⁷ attribute most early twentieth century warming and mid-twentieth century cooling to natural variability; but they assign most late twentieth century warming to anthropogenic (manmade) causes. Regardless of the causes of such changes, controversy over the value of climate sensitivity continues to create problems for public planners and policy makers. Natural climate change has significantly affected human civilization throughout recorded history, but the perceived degree of future climate change—whether natural, anthropogenic, or both—greatly impacts the implementation of mitigation and/or adaptation policies.

A number of recent studies have attempted to address the range of Earth’s climate sensitivity. The Fifth IPCC Assessment Report (2013) puts climate sensitivity in the wide range of 1.5C to 4.5°C (in the Fourth Assessment of 2007, the lower bound was 2°C). More recent estimates of climate sensitivity cluster both within the high end and low end of the given IPCC ranges.

• John Fasullo and Kevin Trenberth⁸ of the National Center for Atmospheric Research in Boulder, Colorado, predict an ECS value near the high end of the IPCC estimate of about 4°C––meaning they believe the globe will warm by four degrees after a doubling of CO₂ starting from the industrial revolution. They based their estimate on computer models with good track records in simulating humidity in the subtropics over a recent 10-year period.
• Climate researcher James Hansen and his colleagues⁹ provided another relatively high estimate suggesting 3°C+/-1°C based on 4 W/m² (four watts per square meter) of CO₂-related radiative forcing (energy imbalance) since the last glacial maximum (compared to the present warm interglacial climate).

However, uncertainties about feedbacks during the last glacial maximum (dust, clouds, etc.) create potentially significant errors for this approach. Hansen has also suggested a 3 to 4°C climate sensitivity based on the Eemian, the last interglacial warm period (which began 130,000 years ago and ended about 115,000 years ago). Another study, based on radiation patterns in climate models, agreed closely with Hansen’s estimates (3.3°C).¹⁰

A Weaker Climate Response?

However, other researchers, such as Michael Schlesinger of the Climate Research Group (University of Illinois), predict a weaker climate response.¹¹ The Schlesinger study (which formed a part of Ring’s report¹²) applied an in-house computer model to analyze historic temperatures changes as a means of narrowing future predictions. This method yielded a 1.5 to 2°C ECS, based on a doubling of CO₂ since preindustrial times. Schlesinger’s conclusions are particularly interesting because only a decade ago he concluded that there was a 70 percent chance that climate sensitivity would exceed the high range of the Fourth IPCC Assessment Report value of 4.5°C.

Several additional researchers have recently concluded that ECS may be less than the Fourth IPCC Assessment’s mean value (3°C):

• Researchers led by Sydney Levitus¹³ suggest an ECS value of 1.5°C for a doubling of CO₂ based on increases in ocean heat content.
• Climatologist Nicholas Lewis’ work¹⁴ produced a value of 1.6°C within a 90 percent confidence interval of 1.2 to 2.2°C.
• Additionally, another study, published in the journal Climate Dynamics,¹⁵ revealed a climate sensitivity of 1.9°C (1.5 to 2.9°C range) based on ocean heat content estimates (using temperatures at depths of 0 to 2,000 meters) and surface temperature observations from 1951–2010.

This last study considered the relationship between ocean heat uptake and long-term climate sensitivity using 32 CMIP (Coupled Model Intercomparison Project) climate model configurations.

Possible Reduced Sensitivity

Other earlier studies have suggested a reduced climate sensitivity compared to estimates provided by the IPCC. In a 2010 Journal of Climate paper, entitled “Why Hasn’t Earth Warmed as Much as Expected?,” researchers led by Stephen Schwartz noted that the 0.8°C increase that has been observed since the late nineteenth century is much less than the 2.1 to 2.4°C increase that might have been expected given previously assumed values of climate sensitivity.¹⁶ They concluded that, over the industrial era, the climate has warmed by about 40 percent of the value expected based on estimates from the 2007 IPCC climate assessment.

Schwartz’s group calculated that natural factors explained only 15 percent of this discrepancy and that thermal imbalances in the Earth’s climate system might account for an additional 25 percent of the difference. Inaccurate estimates of climate sensitivity and uncertainties related to aerosols were identified as possible sources of error. As a result of these findings, Schwartz’s team suggested a possible planetary heating imbalance of 0.37 W/m² or about 2°C of warming by the end of twenty-first century (about half of what has been assumed by high-end ESC estimates).¹⁷ Petr Chylek and his associates (2007) seem to broadly agree with these conclusions, saying that the Earth’s climate sensitivity produces a 0.2 to 0.48 W/m² (watts per square meter) heating rate.

What does this mean? Scientists are not yet able to determine with any unanimity how Earth will respond to an increase of carbon dioxide in the atmosphere that began with the industrial revolution and may double by the end of the twenty-first century. Research continues, but prudence would dictate neither an overreaction to nor a dismissal of the data surrounding Earth’s climate sensitivity.

Part 2 of this series will continue discussing scientist’s efforts to calculate climate sensitivity and what this might indicate regarding the design of the Earth’s climate system.

Dr. Kevin Birdwell

Kevin R. Birdwell received his PhD from the University of Tennessee in 2011 and currently serves as a meteorologist and atmospheric researcher near Knoxville, Tennessee.