A51D-01 INVITED
Modern observations of the Earth's albedo
Broadly speaking, changes in climate depend essentially on three basic parameters, the amount of incident sunlight, the fraction of this sunlight that is reflected by the Earth, and the trapping of the Earth's infrared radiation by clouds and greenhouse gases. While substantial efforts have been made over the past few decades at characterizing the effects of both the solar irradiance and the greenhouse gases on climate, significant questions remain regarding the Earth's reflectance -or albedo. Recent estimates of its variability from a range of sources indicate a much larger-than-expected, and not currently understood, inter-annual and decadal variability. Here, I will focus on the available datasets related to Earth's reflectance, in order to assess the observational constraints on the models. We find a consistent picture among all datasets of an albedo decreased during 1985-2000 between 2-3 and 6-7 W/m2, which is highly climatically significant. The largest discrepancy among the datasets occurs during 2000-2004, when some present an increasing reflectance trend, while others show a steady decrease.
A51D-02 INVITED
Temporal Variation of Global Albedo from CERES Measurements between 2000 and 2005
The Clouds and the Earth's Radiant Energy System (CERES) experiment provides integrated and consistent cloud/aerosol/surface/radiation climate data records sufficiently accurate for scientific investigations of climate forcing and feedbacks. CERES instruments have been operating continuously from December 1999 to present on the Terra spacecraft and from May 2002 to present on the Aqua spacecraft. To produce the CERES monthly data products, the CERES team merges data from up to 11 instruments on 7 spacecraft all integrated to obtain climate accuracy of top-to-bottom atmosphere radiative fluxes in the shortwave and longwave spectral regions. In addition, CERES also incorporates meteorological data from the 4-D weather assimilation models in order to produce its data products. This presentation provides an overview of the CERES project and shows recent results focusing on temporal variations in the Earth's global albedo and an analysis of uncertainties in CERES TOA albedo estimates.
A51D-03
The Darkening of the Earth's Albedo at High Northern Latitiudes During 2006 as Measured by MISR
The deseasonalized anomalies in the time series of globally averaged top-of-atmosphere spectral albedos measured by MISR have now been analyzed from 2000 through 2006. The initial record from 2000 through 2005 showed little in the way of significant anomalies. However, a significant decrease was detected during mid 2006. This anomaly disappeared by the end of 2006 and does not appear to be an instrumental or sampling aberration. The 2006 anomaly is restricted to latitudes north of 40° during late Spring and early Summer, and is large enough to affect the global annual average. The implications of this measurement for sustained ice/snow-albedo feedback will be discussed.
A51D-04
Albedo change by anthropogenic aerosols: Where uncertainty does and does not matter
Modern climate-change theory holds that the relatively stable climate of the Holocene is being altered (or "forced") by human activities. The two primary forcings are heat-trapping by anthropogenic greenhouse gases (GHGs) and albedo increase by anthropogenic aerosols. These have opposing effects on global-mean surface temperature. Thus, accurate quantification of both forcings would seem to be required in order to interpret the causes of temperature change to date and, in turn, assess the validity of climate models and of global warming forecasts. Unfortunately, while human-induced heat-trapping is known to within about 10 percent, human- induced albedo change can only be roughly estimated. As stated in the latest Intergovernmental Panel on Climate Change (IPCC) report, current uncertainties in aerosol forcing lead to a factor-of-four uncertainty in total anthropogenic forcing. However, because GHGs are accumulating while aerosols are not, the relative uncertainty in projected anthropogenic forcing is far less. For example, we will show that IPCC's A1F1 emission scenario (population stability by mid-century with continued economic growth, improvements in efficiency, and reliance on fossil fuels) results in an anthropogenic forcing by 2100 of 9 W/m2 with an uncertainty of only about 20 percent. In short, aerosol forcing uncertainty degrades our abilities to interpret 20th century warming, test climate models, and forecast future warming, but it does not significantly degrade our ability to forecast future forcing. Given this, we suggest that forcing projections be used directly as a gauge of human impact - for example, by comparison to known natural forcings. A forcing of 9 W/m2 is at the upper end of the estimated forcing associated with the previous glacial-to-interglacial transition.
A51D-05
The Annual Cycle of the Energy Budget: Global mean and Land-Ocean Exchanges
The mean and annual cycle of energy flowing into the climate system and its storage, release, and transport in the atmosphere, ocean, and land surface are estimated with recent observations. An emphasis is placed on establishing internally consistent quantitative estimates with a full discussion and assessment of uncertainty. At the top-of-atmosphere (TOA), adjusted Earth Radiation Budget Experiment (ERBE) and Clouds and the Earth's Radiant Energy System (CERES) satellite retrievals are used, while in the atmosphere NCEP/NCAR and ECMWF reanalysis (ERA-40) estimates are used. The net upward surface flux (Fs) over ocean is derived from the residual of TOA and atmospheric budgets, and is compared with direct calculations of ocean heat content (Oe) and its tendency from several ocean temperature datasets. Over land Fs from a stand-alone simulation of the Community Land Model forced by observed fields is used. The near balance between net TOA radiation (R) and Fs over ocean and thus with Oe, and between R and atmospheric total energy divergence over land, are documented both in the mean and for the annual cycle. However, there is an annual mean transport of energy by the atmosphere from ocean to land regions of 2.2±0.1 PW primarily in the northern winter when the transport exceeds 5 PW. The global albedo is dominated by a semiannual cycle over the oceans, but combines with the large annual cycle in solar insolation to produce a peak in absorbed and net radiation in February, somewhat after the perihelion, and with the net radiation 4.3 PW higher than the annual mean, as it is enhanced by the annual cycle of outgoing long-wave radiation that is dominated by land regions. In situ estimates of the annual variation of Oe are found to be unrealistically large.
A51D-06 INVITED
Earth's Albedo in GCMs - Model Performance and Impact of Model Tuning
Albedo estimates from coordinated simulations with 20 different GCMs, performed in support of the IPCC 4th assessment report, and measurements obtained from ERBE (Earth Radiation Budget Experiment) and CERES (Clouds and the Earth's Radiant Energy System) are compared and evaluated. Study of seasonal anomalies, temporal trends and spatial distribution of albedo and model-to-observation correlations, shows that models and observations differ in many aspects. Deviations are especially pronounced in certain regions, for instance marine subtropical areas dominated by stratocumulus clouds. While the models display similar albedo characteristics in terms of e.g. absolute values and geographical distribution, inter-model differences in albedo-determining cloud properties such as cloud water content are large. This indicates that at least some models fail to realistically reproduce cloud-albedo interactions, which limits the extent to which these models can be used to further understand such relationships. The modelled global mean TOA albedo is found to be systematically higher than the observed, and the models deviate more from the more recent CERES measurements than from the older ERBE measurements, most likely as a consequence of being tuned to agree with ERBE TOA fluxes. Tuning the NCAR CAM3.1 (Community Atmosphere Model) to CERES TOA radiative fluxes instead of ERBE is found to give a small but statistically significant difference in the model's equilibrium climate sensitivity. This elucidates the fact that climate sensitivity calculations are indirectly based on parameters that are not well restricted by observations and makes clear the need for more restricting measurements to avoid arbitrariness in climate sensitivity estimates and future climate predictions.
A51D-07
Decadal Variations in Surface Solar Radiation
Satellite estimates provide some information on the amount of solar radiation absorbed by the planet back to the 1980s. The amount of solar radiation reaching the Earth surface can be traced further back in time, untill the 1960s at widespread locations and into the first half of the 20th Century at selected sites. These surface sites suggest significant decadal variations in solar radiation incident at the surface, with indication for a widespread dimming from the 1960s up to the mid 1980s, and a recovery thereafter. Indications for changes in surface solar radiation may also be seen in observatinal records of diurnal temperature range, which provide a better global coverage than the radiation measurrements. Trends in diurnal temperature ranges over global land surfaces show, after decades of decline, a distinct tendency to level off since the mid 1980s. This provides further support for a significant shift in surface solar radiation during the 1980s. There is evidence that the changes in surface solar radiation are linked to associated changes in atmospheric aerosol. Variations in scattering sulfur and absorbing black carbon aerosols are in line with the variations in surface solar radiation. This suggests that at least a part of the variations in surface solar radiation should also be seen in the clear sky planetary albedo. Model simulations with a GCM which includes a sophisticated interactive treatment of aerosols and their emission histories (ECHAM5 HAM), can be used to address this issue. The model is shown to be capable of reproducing the reversal from dimming to brightening under cloud-free conditions in many parts of the world, in line with observational evidence. Associated changes can also be seen in the clear sky planetary albedo, albeit of smaller magnitude.
A51D-08
Solar Constant or Terrestrial Reflectance -- Which Changes More?
The net energy reaching Earth varies over the solar cycle and longer timescales. The net depends on the solar constant and terrestrial reflectance, but which matters more for climate change? Here, we review our current knowledge of variations of the solar constant from ground-based observations, as well as helioseismic and other satellite data. These place tight limits on the range of variability of the historical Sun. On the other hand, earthshine measurements of the Earth's reflectance over a solar cycle imply variations in net energy deposition that are about an order of magnitude larger than those resulting from the changing solar constant. Connections between changes in the solar constant and Earth's reflectance will be discussed to probe the origin of the terrestrial footprint of the solar cycle, as well as the origin of periods like parts of the Maunder Minimum when the Sun was less active and the Earth was cooler.