Climate Sensitivity, Part 2

The determination of climate sensitivity—broadly speaking, the sensitivity of Earth’s climate to changes in energy inputs and outputs—is an important, though tricky area of research. Could scientific knowledge about climate sensitivity suggest something about the design of Earth?

The recent release of the Fifth Assessment Report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC) has reinvigorated global warming discussions. The determination of climate sensitivity is one major issue impacting our understanding of the global climate and, thus, policymakers’ efforts to respond. In part 1 of this series, I discussed various research efforts to determine climate sensitivity, a challenging area of research considering the many factors that influence global temperatures. Here in part 2, I will continue looking at research in climate sensitivity and what our understanding of this subject says about the design of Earth.

Climate Sensitivity Research

Recall from part 1 that 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 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 positive radiative forcing (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 CO₂ concentration.²

A research team led by Magne Aldrin of the Norwegian Computing Center tested climate sensitivity based on a simple climate model fitted to hemispheric temperatures and ocean heat content back to 1850 and 1955, respectively.³ They considered a range of factors including CO₂, methane, nitrous oxides, tropospheric ozone, stratospheric ozone, stratospheric water vapor, direct aerosol effects, cloud albedo/indirect aerosol effects, land use change, solar radiation, and volcanoes. The study yielded an ECS value of 2°C within a 90 percent confidence range of 1.2 to 3.5°C. Aldrin’s group emphasized that both near-surface temperature and ocean heat content are important for estimating climate sensitivity.

One of the lowest estimates of climate sensitivity was produced by climatologists Richard Lindzen and Yong-Sang Choi.⁴ They used observations of sea surface temperature and top-of-the-atmosphere outgoing radiation from 1985 to 2008. Their estimate yielded an ECS of 0.7°C with a 90 percent confidence interval of 0.5 to 1.3°C. This low value may be influenced by nonradiative cloud feedbacks in the tropics, a factor that remains poorly defined due to a limited amount of high-precision cloud and radiative feedback data. Considering that the climate has already warmed about 0.8°C since the beginning of the industrial revolution, a sensitivity of 0.7°C seems insufficient to completely explain future warming, although the value is still within the upper range of a 90 percent confidence interval (1.3°C).

Cloud Feedbacks

Further research into the “give-and-take” between cloud and surface temperature feedbacks is likely needed to improve climate sensitivity estimates in general. Although advancements have been made in defining moisture-related feedbacks, cloud and water vapor feedbacks still contribute significant uncertainty to climate sensitivity estimates and to climate change projections. Lindzen and Choi⁵ point out that most “high” climate sensitivity estimates have been based on climate models that seem to have difficulties in fully accounting for cloud feedbacks.

In 2011, another team further suggested that signals related to low clouds may represent up to half of this cloud feedback problem.⁶ Others, like Lindzen and Choi, describe the issue in terms of a misinterpretation of the covariability of clouds and temperature when specifying cloud parameterizations.⁷ That is, clouds cause chaotic radiative forcing of temperature, which results in the false appearance of a positive feedback, potentially confusing cause and effect. Lindzen and Choi⁸ further noted that the atmosphere and ocean are too weakly coupled in many current climate models and, as a result, do not adequately model the transfers of energy through the climate system.

Ocean Heat Content

If climate models in general do not adequately account for the energy transfers between the atmosphere and the oceans, better accounting of how much heat the ocean retains may  explain some of the discrepancies between high and low climate sensitivity estimates. The slow turnover rate of currents in the deep ocean suggests that the overall response of the ocean to changes in energy inputs may also occur slowly. Even with the major improvement in measurements of ocean salinity and temperature since the early 2000s (supplied by an observation network known as Argo, named after the mythical Greek ship), there remains a lack of understanding of the deep ocean currents and vertical mixing properties, particularly below a depth of 2,000 meters.

Although much debate remains about their findings, researchers led by Magdalena Balmaseda have suggested that the low climate sensitivity observed during the 2000s—which manifested as either a lack of or significant slowing of warming despite large greenhouse gas emissions to the atmosphere—could be associated with heat transfer into the deep ocean.⁹ However, if the deep ocean acts as such a heat sink, then the process must necessarily occur slowly. Conversely, the near-surface ocean, encompassing the ocean’s top 50 to 100 meters, responds relatively quickly to energy inputs because it is well-mixed by wind and atmospheric influences. Thus, the near-surface ocean acts as a single heated layer. Just as atmospheric inputs affect the ocean surface layer, the surface ocean typically has the most influence on the atmosphere within time scales of months to a few years.

In support of their view, Balmaseda’s group noted an increase in deep-ocean heat content after 2000. However, this change in heat content coincided with a change to a cool phase of the Pacific Decadal Oscillation (PDO). The PDO is a natural variation in ocean temperatures in the Pacific Ocean that typically persists for a few decades in either a warm or cool phase. The coincidence of this heat content change with a change in PDO phase may imply the influence of natural climate variation. Uncertainty regarding natural fluctuations in ocean heat content absorption can significantly modify the projections of climate response to a given radiative forcing.8 In other words, it is not well-understood how these natural variations in ocean-scale climate modify changes in atmospheric climate that result from human-induced greenhouse gas emissions and other factors. However, if the PDO cool phase has slowed global warming since 2000, this implies that accelerated global warming that occurred during the 1980s and 1990s may have been enhanced by the naturally-occurring PDO, when the PDO was in a warm phase.

The Future of Climate Sensitivity Research

The definitions of climate sensitivity described in this series do not always provide the best results for defining the climate’s response to changes in radiative inputs, especially since these calculations may not consider all natural and anthropogenic feedbacks. However, these estimates do provide a baseline for future assessment and study. Most high-end climate sensitivities discussed here have been based on computer models or on paleoclimate data. Conversely, many of the low-sensitivity estimates were derived from observation-based data. Uncertainties plague both types of climate sensitivity estimates. For example, problems with estimating model-based parameterizations introduce error for some of the high-end estimates while difficulties in distinguishing the magnitude of individual feedbacks that affect observational data may cause errors for the low-end estimates. However, lower climate sensitivity estimates (observations) tend to better match the historical record thus far, which generally predict an ECS of 1.5 to 2°C for a doubling of CO₂. Not that the IPCC encompassed most of the high and low-end climate sensitivity estimates within its range of 1.5 to 4.5°C).

Global temperature changes ranging from 1 to 2°C have caused significant prehistorical and historical disruptions of human activities, particularly with regard to agriculture and fisheries. These changes have impacted regional landscapes, climate, and atmospheric circulation patterns that influence temperature, precipitation, winds, cloud cover, and other factors. During the last interglacial period (Eemian), temperature increases of 2°C or more in some parts of the world were associated with a warmer Arctic, sea levels that reached six meters above the present, and changes in the geography of climate zones (for example, a subtropical climate in England). However, a warming of 1.5 to 2°C from pre-industrial times to the late 21st century, which currently seems supported by observations, would be more preferable and manageable than that of 4°C or more warming predicted by some.

If the deep ocean does play a role in absorbing extra heat inputs, especially with regard to high climate sensitivity, an element of design is implied because the deep ocean acts to slow climate change, particularly within the current setting of the present interglacial climate associated with the current global continental and ocean geography. However, if hypothesized changes in ocean heat content are related mostly to natural decadal cycles such as the PDO, then the long-term uptake of anthropogenically-produced heat into the ocean may be occurring more slowly than suggested by Balmaseda’s team and others, implying a lower climate sensitivity, and thus suggested a measure of design in that there is a built-in resistance to major changes in climate.

In Job 38:16, God revealed to Job that he knew little about the workings of the ocean depths. Our knowledge today, though still limited, has improved since Job’s time. We are discovering that the ocean is a great climate stabilizer that makes Earth more habitable than it otherwise would be. This does not suggest that the issue of climate change should be trivialized because of such stabilizing influences. Significant natural climate change has occurred in the past and will occur in the future with or without human help. However, if the global climate is to be accurately predicted and managed, a better understanding of deep ocean processes and these influences on climate feedbacks is needed. As scientists continue to study, all humanity stands to benefit from a better understanding that can lead to improved stewardship and greater appreciation for the inner workings of a designed 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.