Much of our recent research has focused on the radiative influence of anthropogenic aerosols on climate in connection with the larger issue of possible human-induced climate change, the so-called global warming issue (139K pdf file). Aerosols affect the earth's radiation budget directly, by scattering incoming shortwave (solar) radiation and thereby enhancing the earth's albedo, and indirectly, by modifying the microphysical properties and reflectivity of clouds. We and others have presented a body of work over the past decade that indicates that anthropogenic aerosols are exerting an influence on climate change that is comparable (but of opposite sign) to the anthropogenic greenhouse effect. However the magnitude of these aerosol influences is quite uncertain in comparison to that of longwave (thermal infrared) radiative forcing by incremental concentrations of greenhouse gases (mainly carbon dioxide and to lesser extent methane, nitrous oxide, and others) resulting from industrial activity.

Estimates from the IPCC (Intergovernmental Panel on Climate Change) Report Climate Change 2007 -- The Physical Science Basis on of the several contributions to radiative forcing over the industrial period are shown in the following figure. (The unit of forcing, and of energy fluxes in geophysics generally, is the watt per square meter.) Also shown (as "I-beams") are the IPCC's estimates of the uncertainties associated with each of these quantities. It is seen that the uncertainties of the several aerosol forcings substantially exceed those associated with the greenhouse gases and other forcings; with the 2007 report the IPCC for the first time provided an estimate for the first indirect forcing (cloud albedo enhancement), rather than giving only an indication of the range of possible values as in the 2001 report. Added to the figure (light blue bar) is an estimate of the total (direct plus first indirect) forcing, -1.2 W m^-2, and the associated uncertainty range: -0.6 to -2.4 W m^-2.

Importantly, also for the first time in the 2007 report the IPCC Working Group provided an estimate of the total forcing over the industrial period and of the associated uncertainty, based on the assumption, which is inherent in the forcing-response hypothesis, of additivity of forcings, as we have previously advocated [Schwartz and Andreae, Science, 1996; Schwartz, J. Air Waste Management Assoc., 2004]. It is seen that the uncertainty range in the estimated total forcing is quite large. This uncertainty range is due almost entirely to the uncertainty in aerosol forcing; if the aerosol forcing is small (negative) the total forcing is large, 2.4 W m^-2, whereas if the aerosol forcing is large (negative) this negative forcing is offsetting a major fraction of the positive forcing (mainly greenhouse gas forcing) and the total forcing is at the small end of the uncertainty range, 0.6 W m^-2. The central 90% confidence limits of the estimated forcing differ by a factor of 4.

It should be emphasized that one should not take any comfort with the fact that the aerosols may be negating much of the greenhouse gas forcing--in fact just the opposite. Because the atmospheric residence time of tropospheric aerosols is short (about a week) compared to the decades-to-centuries lifetimes of the greenhouse gases, then to whatever extent greenhouse gas forcing is being offset by aerosol forcing, it is last week's aerosols that are offsetting forcing by decades worth of greenhouse gases. Because the greenhouse gases are long-lived in the atmosphere, their atmospheric loadings tend to approximate the integral of emissions. Because the aerosols are short-lived, their loading tend to be proportional to the emissions themselves. There is only one function that is proportional to its own integral, the exponential function. So only if society is to make a commitment to continued exponential growth of emissions can such an offset be maintained indefinitely. And of course exponential growth cannot be maintained forever. So if the cooling influence of aerosols is in fact offsetting much of the warming influence of anthropogenic greenhouse gases, then when society is unable to maintain this exponential growth, the climate could be in for a real and long-lasting shock.

Why is it essential to reduce uncertainty in radiative forcing over the industrial period? To my thinking the primary reason is to improve understanding of climate change over this period, more specifically to reduce uncertainty in climate sensitivity, the amount by which the global mean surface temperature, GMST, would change in response to a given forcing. Knowledge of the climate forcing over the industrial period is essential to empirical determination of Earth's climate sensitivity, from the observed increase in GMST over the instrumental record, together with forcing over the same period, or alternatively, as input to calculations with global climate models, GCMs, necessary to evaluate the performance of these models over the period of instrumental record.

A consequence of uncertainty in forcing over the instrumental record as input to climate model calculations is seen in the comparison of modeled and observed temperature change over the twentieth century, as reported in the 2007 IPCC Assessment Report. The figure below, modified from the figure that originally appeared in the report, compares observed changes in global mean surface temperature with results simulated by 14 different climate models using natural and anthropogenic forcings. The modeled change in global mean surface temperature exhibits a spread of less than a factor of 2, well less than the factor of 4 spread in the estimates of radiative forcing over the industrial period discussed above. In our paper [Schwartz et al. Nature Reports on Climate Change (2007)] we asked how it could be that the spread in modeled temperature change was well less than that in the forcing that was necessary as input to the model, speculating that one possible reason might be anticorrelation between the sensitivities of the several models used in the study and the forcings employed in the individual studies. This was subsequently shown to be the case for a subset of the models [Kiehl, Geophysical Research Letters, 2007]. This situation certainly suggests a much greater uncertainty in climate sensitivity than might be inferred from the agreement between modeled and observed temperature change over the twentieth century as presented in the IPCC report.

Observed and modeled change in global mean surface temperature over the twentieth century. Decadal averages of observations are shown for the period 1906-2005 (black) plotted against the center of the decade and relative to the corresponding average for 1901-1950. The blue band shows the 5-95% range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. The rose-colored band shows the 5-95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. Added to the figure are I-beams denoting uncertainties. The range of the modeled increase in global mean surface temperature over the twentieth century (red) - ~0.5 to 1.0 K, or a factor of 2 - is well less than that of the IPCC estimate for the global mean forcing, a factor of 4 (green). Modified from the IPCC 2007 Assessment report. From Schwartz et al. Nature Reports on Climate Change (2007).

I would argue that knowledge of Earth's climate sensitivity is of enormous importance to the peoples of the world and to planning the means by which to meet collective future energy requirements. At present over 85% of current primary energy derives from combustion of fossil fuels, which results in emission of CO₂ into the atmosphere, where it accumulates, with a lifetime of the excess CO₂ of something like 100 years. This excess CO₂ will thus continue to exert a radiative forcing over this time period, and as further CO₂ is emitted in the future, the mixing ratio of atmospheric CO₂ and the resultant radiative forcing will continue to increase. The key question therefore is what will be the resultant changes in Earth's climate, and the single most important index of such change is the increase in global mean surface temperature. Hence the need to determine Earth's climate sensitivity.

The sensitivity of Earth's climate to perturbation in radiative flux has been of interest to climate scientists for quite some time. Historically this sensitivity has been expressed as the equilibrium increase in global mean surface temperature GMST that would result from a doubling of the mixing ratio of CO₂ in the atmosphere ΔT_2×. This measure of climate sensitivity has been commonly used as a benchmark for examining sensitivity in climate models. However absent a major decrease in emissions of CO₂, mainly from fossil fuel combustion, a doubling of atmospheric CO₂ will occur well before the end of the present century. For this reason Earth's climate sensitivity takes on a significance to the peoples of the world that goes well beyond a measure of the sensitivity of climate models.

A history of estimates of Earth's climate sensitivity is shown in the following figure. Here climate sensitivity is expressed on the right axis as the change in GMST that would result from a change in radiative flux of 1 W m^-2, and also, on the left axis, as ΔT_2×; the conversion from K/(W m^-2) to ΔT_2× assumes a forcing of doubled CO₂ F_2× equal to 3.7 W m^-2. To my thinking the unit W m^-2 for climate sensitivity is preferable, as it does not rely on any particular value of F_2×, which quantity differs substantially even among current climate models.

Estimates of Earth's equilibrium climate sensitivity. Estimates are expressed as the increase in global mean surface temperature GMST that would result from a doubling of the amount of CO₂ in the atmosphere ΔT_2×, left axis, or as the change in GMST that would result from a change in radiative flux of 1 W m^-2, right axis; the conversion from ΔT_2× to K/(W m^-2) assumes a forcing of doubled CO₂ F_2× equal to 3.7 W m^-2. The point denoted "Stefan-Boltzmann" is the sensitivity that would apply to a black body radiator at Earth's mean surface temperature, 288 K. The point denoted "Arrhenius" was based on a calculation that accounted for water vapor and snow-ice feedback as a function of latitude and season. The National Research Council "Charney Report" (1979) gave a best estimate ΔT_2× of 3 K with uncertainty range ± 1.5 K. The remaining estimates are from successive assessment reports of the International Panel on Climate Change (IPCC), the organization which, together with Al Gore, was awarded the 2007 Nobel Peace Prize; the first three reports gave only an estimated range; the most recent, 2007, report provides a best estimate ΔT_2× of 3 K with a slightly decreased uncertainty range, 2 - 4.5 K. The notations "1 sigma", "Likely", and "> 66%" denote the likelihood that the actual climate sensitivity lies within the indicated uncertainty range.

The estimate of climate sensitivity denoted "Stefan-Boltzmann," which is obtained by application of Stefan's law for the temperature dependence of the radiative flux of a black body, gives the sensitivity for a black body radiator at Earth's GMST of 288 K (15 °C; 59 °F). To my knowledge Stefan did not actually calculate Earth's climate sensitivity, although he might have used his radiation law to do so; he did use his formula to obtain a very accurate determination of the temperature of the Sun. The next estimate shown is that of Arrhenius (same Arrhenius who gave us the theory of ionic solutions and the activation energy of chemical kinetics) who made what we would now call a "spread sheet" calculation (except that he did not have a personal computer with which to evaluate the entries) that took into account the feedback from water vapor and snow-ice albedo as a function of latitude and season. Any increase in sensitivity over that of a black body is due to positive feedbacks inherent in the model of the climate system (or for that matter in Earth's actual climate system). Arrhenius thought that global warming from the increase in atmospheric CO₂ due to fossil fuel combustion would be a benefit to the cold climate of Sweden.

The several remaining estimates of climate sensitivity derive from major national or international assessments, a 1979 report by a panel of the National Academy of Sciences headed by Jule Charney, and the four assessment reports of the International Panel on Climate Change, IPCC. The Charney report gave a best estimate of the climate sensitivity as ΔT_2× = 3 ± 1.5 K. The first three IPCC reports declined to give a best estimate of Earth's climate sensitivity but presented only a likely range for this quantity. The most recent (2007) report again presents a best estimate, which coincides with that of the 1979 Charney report, and slightly decreases the estimate of the lower uncertainty range.

In view of the expected increase in radiative forcing that may be expected over the present century due to increases in CO₂ and other forcing agents the magnitude of Earth's climate sensitivity indicated in the above figure assumes a significance to human society that transcends mere scientific interest. The increase in GMST of 3 K, corresponding to the best current estimate of sensitivity might be compared to the change in GMST of perhaps 6 K between the present temperate era and the last glacial maximum, which was characterized by a kilometer-thick ice sheet over much of central North America. (Long Island, where I live and work, is the terminal moraine of that ice sheet.) Not much of a stretch of the imagination is required to envision that an increase in GMST of 3 K (or 4.5 K if the sensitivity is at the high end of the uncertainty range) could result in the melting of the last remnant of the North American ice sheet, that is, the Greenland ice sheet. Such a melting would increase global sea level by 7 meters, with resultant loss of much inhabited land and property of enormous economic and cultural value. The social and political consequences of such a rise in sea level would be enormous.

In view of the above considerations the present uncertainty in climate sensitivity assumes great importance to the peoples of the world. Assume, consistent with the United Nations Framework Convention on Climate Change, that it is decided to limit the increase in global mean surface temperature to some agreed upon value to avoid dangerous anthropogenic interference with the climate system. The present uncertainty in climate sensitivity of at least a factor of 2, and perhaps more, corresponds to a like uncertainty in the amount of incremental radiative forcing that would be consonant with such an increase in GMST. This in turn corresponds to an uncertainty in the amount of increase in atmospheric CO₂, δCO₂, that would be consonant with such an agreed upon bound to the increase in GMST (within the approximation that ln(1 + δCO₂) = δCO₂, the forcing by CO₂ being roughly proportional to the logarithm of the mixing ratio). Thus the uncertainty in climate sensitivity translates directly into a like uncertainty in the shared global resource of the amount of fossil fuel carbon that can be combusted and introduced into the atmosphere consonant with a given increase in global temperature. Knowledge of the climate sensitivity is thus of enormous value to planning the nation's and the world's energy future.

Recently I have introduced an alternative empirical approach to determining Earth's climate sensitivity by means of a single-component energy balance model. The basis of the approach is the recognition, within such a model, that the climate sensitivity S is related to the time constant for Earth's climate system to respond to a perturbation τ and the effective heat capacity of the climate system C as S = τ/C. When I present this work in a lecture I observe that this is one equation in three unknowns! What I proceeded to do in my study was first to determine the effective heat capacity of the climate system as the ratio of the slopes with time t over the instrumental record of global heat content H, which is dominated by ocean heat content, and global mean surface temperature T, C = (dH/dt)/(dT/dt). I then determined the time constant of the climate system from the decorrelation time of fluctuations of global mean surface temperature, a relation that goes back to Einstein's fluctuation-dissipation theorem. In my initial paper (Schwartz, 2007) I obtained a time constant 5 ± 1 yr, that was somewhat too low, because, as was pointed out in an about to be published (summer, 2008) Comment on my paper, there is a second, much shorter time constant, about 0.4 yr, that confounds the analysis. My revised analysis, about to be published (summer, 2008), yields a time constant of 8.5 ± 2.5 yr. This in turn results in a climate sensitivity of 0.51 ± 0.26 K/(W m^-2), corresponding to an equilibrium temperature increase for doubled CO₂ of 1.9 ± 1.0 K, somewhat lower than the central estimate of the sensitivity given in the 2007 assessment report of the Intergovernmental Panel on Climate Change, but consistent within the uncertainties of both estimates. The relatively short time constant of the climate system means that the departure of the current increase in GMST from that which would be expected if the system were at equilibrium is quite small. This study is quite controversial and has drawn three about to be published Comments as well as much discussion on the Web. It also drew considerable media attention. I believe much of the attention resulted from the quite low climate sensitivity in the initial paper. I am hopeful that the work I have presented will stimulate further research along these lines to better circumscribe the limits to this approach and perhaps to more tightly constrain climate sensitivity than I have been able to do thus far.

Because uncertainties associated with aerosol forcing are the major source of uncertainty in climate forcing over the industrial period, it is essential in my opinion to focus on the aerosol forcing to advance quantitative understanding of anthropogenically induced climate change. Consequently much of our research is directed to developing such improved understanding.

In several studies we have tried to provide estimates of the uncertainty budget associated with the aerosol forcing. Much of the uncertainty arises from the fact that unlike the long-lived greenhouse gases, whose concentrations are rather uniformly distributed in the atmosphere, the loadings of aerosols are highly variable in space and time, as a consequence of highly localized sources and of sporadic removal, mainly by precipitation. Additionally aerosol microphysical properties are not a universal constant, but depend on sources and composition and evolve as a consequence of chemical and physical processes occurring in the atmosphere. The mass loading, composition, and the microphysical properties of aerosols such as number concentration and size distribution directly affect their direct and indirect radiative forcing of climate.

Reducing the uncertainty in aerosol forcing will require a major effort both in characterizing the present distribution and properties of aerosols and in developing understanding required to represent the processes controlling loading and properties of tropospheric aerosols in numerical models. Model-based descriptions of aerosol forcing need to be incorporated into climate models in order to represent this forcing not just for the present climate but also retrospectively over the industrial period and prospectively for various scenarios of future emissions. Much of our research is directed to developing and evaluating numerical models for representing the geographical distribution of loading of atmospheric aerosols. Our approach has been to use observationally derived meteorological data to drive our models, because the temporal and spatial variation in aerosol loading is governed to great extent by meteorological variability. Meaningful evaluation of the model by comparison with observations thus requires this approach. Much of this work is conducted within the Department of Energy's Atmospheric Science Program (ASP).

An additional major component of our research is directed to developing improved representation of aerosol optical properties and radiative forcing. Much of this work is conducted in conjunction with the Department of Energy's Atmospheric Radiation Measurement (ARM) Program.

Our work is represented in our publications. I welcome inquiries of interest from any and all. Much of our work is conducted in collaboration with others at their institutions or as visiting scientists at Brookhaven National Laboratory. I particularly encourage inquiries from students; you are our future.

A riddle that's gained some currency in our group: "What's black and white and red all over?" Answer

The batting average paradox. Able has a higher batting average than Baker in the first half of the season and also in the second half. You might think that that means that Able has a higher average for the season. But you would be wrong. Click here to see why averaging ratios can be misleading.

The DOE Office of Science 1999 Strategic Plan included a section on the climate influence of aerosols, highlighting our research. To download just this section (600 K pdf file) click here.

Modeling atmospheric sulfate on subhemispheric to hemispheric scale. Our group has developed (and continues to develop) a chemical transport model for atmospheric sulfate and precursor species. The output of this model is mixing ratios of sulfate as a function of location and time. The model is driven by observationally derived meteorological data (archived numerical weather prediction forecast model results) so the modeled sulfate may be compared with observations at specific locations and times.

Animation of the model results brings out features of the calculations and their temporal evolution that are not readily discernible from static images. We have pioneered in publication of such animations in a peer-reviewed electronic journal.

Dynamical influences on the distribution and loading of SO₂ and sulfate over North America, the North Atlantic and Europe in April 1987. Benkovitz C. M., Miller M. A., Schwartz S. E. and Kwon O-U. Geochem. Geophys. Geosyst. 2, Paper no. 2000GC000129 (2001). http://www.agu.org/journals/gc/gc0106/2000GC000129/fs2000GC000129.html.

Modeling atmospheric sulfate on subhemispheric scale. This link connects to Quicktime (R) movies generated with output from our model showing the evolution of column burden of atmospheric sulfate (vertical integral of concentration) over several one-month periods.

Modeling sulfate on hemispheric scale. This link connects to movies generated with output from our model showing the evolution of column burden of sulfate from Asian anthropogenic sources and of total sulfate as a function of location in the Northern Hemisphere, June-July 1997.

Modeling volcanic sulfate on hemispheric scale. This link connects to a movie generated with output from our model showing the temporal and spatial evolution of column burden of sulfate from volcanic SO₂ emissions in the Northern Hemisphere, June-July 1997.

Aerosol Perturbations on Climate. Our model runs on computers at NERSC (National Energy Research Scientific Computing Center). Our work was featured in the climate modeling section of the 1998 NERSC Annual Report .

Grains of Salts. An account of our work presented in a plenary lecture at the 1997 annual meeting of the American Association of Aerosol Research was featured in the February 1998 issue of ER News, Newsletter of the Department of Energy's Office of Energy Research.

High-Resolution Model for Tropospheric Sulfate. Our model for tropospheric sulfate was highlighted in a DOE Research Summary (November 1994).

Atmospheric Heating and Cooling from Fossil-Fuel Combustion. Our examination of the greenhouse heating influence of fossil fuel combustion versus the aerosol cooling influence was highlighted in the Fall 1994 Newsletter of the DOE Carbon Dioxide Information and Analysis Center CDIAC Communications.

IN THE NEWS

A sampling of recent news articles about our work

Climate Change: Another Global Warming Icon Comes Under Attack, Science, July 06, 2007

Things Are Heating Up, The North Shore Sun, September 06, 2007

The Aerosol Man, The National Post, September 01, 2007

This page was last updated 2008-07-20.

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