On the Correlation between Forcing and Climate Sensitivity

Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/)The possible correlation between climate sensitivity and radiative forcing is studied using versions of the NCAR Community Atmospheric Model (CAM) model with different climate sensitivities. No such correlation was found for the CO2 forcing. A weak correlation for the direct sulfate aerosol forcing is associated with differences in cloud cover in control climate simulations with different versions of the model. Presented results suggest that correlation between sensitivity and radiative forcing in the 20th century simulations with different AOGCMs is not a reflection of physical reality but is a result of different treatments of forcing agents, primarily aerosols.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors

Does Model Sensitivity to Changes in CO2 Provide a Measure of Sensitivity to the Forcing of Different Nature?

Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).Simulation of both the climate of the 20th century and of possible future climate change requires taking into account numerous forcings of different nature. Climate sensitivities of existing general circulation models, defined as the equilibrium surface warming due to increase in atmospheric CO2 concentrations, vary over a rather wide range. A large number of simulations with the MIT climate model of intermediate complexity with forcings of different nature have been carried out to study to what extent sensitivity to changes in CO2 concentration represent sensitivities to other forcings. Sensitivity of the MIT model can be changed by changing the strength of the cloud feedback. Simulations with the versions of the model with different sensitivities show that the sensitivity to changes in CO2 concentration provides a reasonably good measure of the model sensitivity to other forcings with similar vertical stratifications. However the range of models’ responses to the forcings leading to the cooling of the surface is narrower than the range of models’ responses to the forcings leading to warming. This is explained by the cloud feedback being less efficient in the case of increasing sea ice extent and snow cover. The range of models’ responses to the forcings with different vertical structure, such as increase in black carbon concentration, is also smaller than that for changes in CO2 concentration

Sensitivity of Climate Change Projections to Uncertainties in the Estimates of Observed Changes in Deep-Ocean Heat Content

Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).The MIT 2D climate model is used to make probabilistic projections for changes in global mean surface temperature and for thermosteric sea level rise under a variety of forcing scenarios. The uncertainties in climate sensitivity and rate of heat uptake by the deep ocean are quantified by using the probability distributions derived from observed 20th century temperature changes. The impact on climate change projections of using the smallest and largest estimates of 20th century deep ocean warming is explored. The impact is large in the case of global mean thermosteric sea level rise. In the MIT reference ("business as usual") scenario the median rise by 2100 is 27 and 43 cm in the respective cases. The impact on increases in global mean surface air temperature is more modest, 4.9 C and 3.9 C in the two respective cases, because of the correlation between climate sensitivity and ocean heat uptake required by 20th century surface and upper air temperature changes. The results are also compared with the projections made by the IPCC AR4's multi-model ensemble for several of the SRES scenarios. The multi-model projections are more consistent with the MIT projections based on the largest estimate of ocean warming. However the range for the rate of heat uptake by the ocean suggested by the lowest estimate of ocean warming is more consistent with the range suggested by the 20th century changes in surface and upper air temperatures, combined with expert prior for climate sensitivity.This work was supported in part by the Office of Science (BER), U.S. Dept. of Energy Grant No. DE-FG02-93ER61677, NSF, and by the MIT Joint Program on the Science and Policy of Global Change

Implementation of a Cloud Radiative Adjustment Method to Change the Climate Sensitivity of CAM3

Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).Conducting probabilistic climate projections with a particular climate model requires the ability to vary the model’s characteristics, such as its climate sensitivity. In this study, we implement and validate a method to change the climate sensitivity of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 3 (CAM3) through a cloud radiative adjustment. Results show that the cloud radiative adjustment method does not lead to physically unrealistic changes in the model’s response to an external forcing, such as doubling CO2 concentrations or increasing sulfate aerosol concentrations. Furthermore, this method has some advantages compared to the traditional perturbed physics approach. In particular, the cloud radiative adjustment method can produce any value of climate sensitivity within the wide range of uncertainty based on the observed 20th century climate change. As a consequence, this method allows Monte Carlo type probabilistic climate forecasts to be conducted where values of uncertain parameters not only cover the whole uncertainty range, but cover it homogeneously. Unlike the perturbed physics approach which can produce several versions of a model with the same climate sensitivity but with very different regional patterns of change, the cloud radiative adjustment method can only produce one version of the model with a specific climate sensitivity. As such, a limitation of this method is that it cannot cover the full uncertainty in regional patterns of climate change.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors

What GHG Concentration Targets are Reachable in this Century?

We offer simulations that help to understand the relationship between GHG emissions and concentrations, and the relative role of long-lived (e.g., CO2) and short-lived (e.g., CH4) emissions. We show that, absent technologies to remove CO2 from the atmosphere, the 350 CO2 ppm target is out of reach in this century, even if all emissions drop to zero almost immediately (i.e. in 2015). A 350 ppm CO2-equivalent target is potentially achievable, but would require CH4 concentrations falling below preindustrial levels, and thus elimination of emissions from human activities such as rice and livestock agriculture. More realistically, even some of the most aggressive targets proposed through 2035 would lead to concentrations (CO2 or CO2-eq) in the 415–450 ppm range. This is only feasible if after 2035 emissions continued a downward path toward zero. Only in these cases would the temperature target of no more than 2 °C above preindustrial be achieved, and only after peaking above that level before declining.The MIT Integrated Global System Model (IGSM) and its economic component used in the analysis, the MIT Emissions Prediction and Policy Analysis (EPPA) model, are supported by a consortium of government, industry, and foundation sponsors of the MIT Joint Program on the Science and Policy of Global Change, including U.S. Department of Energy, Office of Science (DE-FG02-94ER61937). (For a complete list of sponsors, see: http://globalchange.mit.edu/sponsors/all)

A Global Land System Framework for Integrated Climate-Change Assessments

Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).Land ecosystems play a major role in the global cycles of energy, water, carbon and nutrients. A Global Land System (GLS) framework has been developed for the Integrated Global Systems Model Version 2 (IGSM2) to simulate the coupled biogeophysics and biogeochemistry of these ecosystems, as well as the interactions of these terrestrial processes with the climate system. The GLS framework has resolved a number of water and energy cycling deficiencies and inconsistencies introduced in IGSM1. In addition, a new representation of global land cover and classification as well as soil characteristics has been employed that ensures a consistent description of the global land surface amongst all the land components of the IGSM2. Under this new land cover classification system, GLS is run for a mosaic of land cover types within a latitudinal band defined by the IGSM2 atmosphere dynamics and chemistry sub-model. The GLS shows notable improvements in the representation of land fluxes and states of water and energy over the previous treatment of land processes in the IGSM1. In addition, the zonal features of simulated carbon fluxes as well as key trace gas emissions of methane and nitrous oxide are comparable to estimates based on higher resolution models constrained by observed climate forcing. Given this, the GLS framework represents a key advance in the ability of the IGSM to faithfully represent coupled terrestrial processes to the climate system, and is well poised to support more robust two-way feedbacks of natural and managed hydrologic and ecologic systems with the climate and socio-economic components of the IGSM2.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors

Constraining Climate Model Parameters from Observed 20th Century Changes

Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).We present revised probability density functions for climate model parameters (effective climate sensitivity, the rate of deep-ocean heat uptake, and the strength of the net aerosol forcing) that are based on climate change observations from the 20th century. First, we compare observed changes in surface, upper-air, and deep-ocean temperature changes against simulations of 20th century climate in which the climate model parameters were systematically varied. The estimated 90% range of climate sensitivity is 2.0 to 5.0 K. The net aerosol forcing strength for the 1980s has 90% bounds of -0.70 to -0.27 W/m2. The rate of deep-ocean heat uptake corresponds to an effective diffusivity, Kv, with a 90% range of 0.04 to 4.1 cm2/s. Second, we estimate the effective climate sensitivity and rate of deep-ocean heat uptake for 11 of the IPCC AR4 AOGCMs. By comparing against the acceptable combinations inferred by the observations, we conclude that the rate of deep-ocean heat uptake for the majority of AOGCMs lie above the observationally based median value. This implies a bias in the predictions inferred from the IPCC models alone. This bias can be seen in the range of transient climate response from the AOGCMs as compared to that from the observational constraints.This work was supported in part by the Office of Science (BER), U.S. Dept. of Energy Grant No. DE-FG02-93ER61677, NSF, and by the MIT Joint Program on the Science and Policy of Global Change

Estimated PDFs of Climate System Properties Including Natural and Anthropogenic Forcings

Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).We present revised probability density functions (PDF) for climate system properties (climate sensitivity, rate of deep-ocean heat uptake, and the net aerosol forcing strength) that include the effect on 20th century temperature changes of natural as well as anthropogenic forcings. The additional natural forcings, primarily the cooling by volcanic eruptions, affect the PDF by requiring a higher climate sensitivity and a lower rate of deep-ocean heat uptake to reproduce the observed temperature changes. The estimated 90% range of climate sensitivity is 2.4 to 9.2 K. The net aerosol forcing strength for the 1980s decade shifted towards positive values to compensate for the now included volcanic forcing with 90% bounds of -0.7 to -0.16 W/m2. The rate of deep-ocean heat uptake is also reduced with the effective diffusivity, Kv, ranging from 0.25 to 7.3 cm2/s. This upper bound implies that many coupled atmosphere-ocean GCMs mix heat into the deep ocean (below the mixed layer) too efficiently.This work was supported in part by the NOAA Climate Change Data and Detection Program with support from DOE, the Joint Program on the Science and Policy of Global Change at MIT, and the Office of Science (BER) DOE Grant No.DE-FG02-93ER61677