519 publications from this institution
This report reflects the discussions and recommendations of Breakout Group B. It is recognized that there are gaps and imbalances in the topics covered. These came about from the particular expertise of the collection of scientists who happened to be present and the...
The monthly global and regional variability in Earth's radiation balance is examined using correlations and regressions between atmospheric temperatures and water vapor with top‐of‐atmosphere outgoing longwave (OLR), absorbed shortwave (ASR), and net radiation ( R T = ASR − OLR). Anomalous global mean monthly variability in the net radiation is surprisingly large, often more than ±1 W m −2 , and arises mainly from clouds and transient weather systems. Relationships are strongest and positive between OLR and temperatures, especially over land for tropospheric temperatures, except in the deep tropics where high sea surface temperatures are associated with deep convection, high cold cloud tops and thus less OLR but also less ASR. Tropospheric vertically averaged temperatures (surface = 150 hPa) are thus negatively correlated globally with net radiation (−0.57), implying 2.18 ± 0.10 W m −2 extra net radiation to space for 1°C increase in temperature. Water vapor is positively correlated with tropospheric temperatures and thus also negatively correlated with net radiation; however, when the temperature dependency of water vapor is statistically removed, a significant positive feedback between water vapor and net radiation is revealed globally with 0.87 W m −2 less OLR to space per millimeter of total column water vapor. The regression coefficient between global R T and tropospheric temperature becomes −2.98 W m −2 K −1 if water vapor effects are removed, slightly less than expected from blackbody radiation (−3.2 W m −2 K −1 ), suggesting a positive feedback from clouds and other processes. Robust regional structures provide additional physical insights. The observational record is too short, weather noise too great, and forcing too small to make reliable estimates of climate sensitivity.
No abstract is provided for this article.
Current human-induced climate change arises primarily from the heating of the planet mainly from changes in atmospheric composition, and temperature change is one manifestation. The increasing greenhouse gases, notably carbon dioxide from burning fossil fuels, lead to Earth’s Energy Imbalance (EEI), altering the flow of energy through the climate system, and the dissemination of excess energy is partly what determines how climate change is manifested. Some of the extremes being experienced, especially those involving drought, convection, storms, flooding, and the water cycle, are mostly driven by aspects of heating and, while temperature contributes through the water-holding capacity of the atmosphere, it is more a consequence than a cause. Afterall, water is the air conditioner of the planet. The United Nations, and especially the Intergovernmental Panel on Climate Change (IPCC) in their Summary for Policy Makers, focus on global temperature targets rather than broader facets of climate change including EEI, and do not always adequately discriminate between temperature and heating. This also has consequences for future climate if or when heating is brought under control by cutting emissions. Improvements are needed in expressing how the climate is changing by properly accounting for the flow of energy through the climate system.
Kevin E. Trenberth emphasizes the fundamental role of energy flows in the climate system and anthropogenic climate change. The distribution of heat, or more generally, energy, is the main determinant of weather patterns in the atmosphere and their impacts. The topics addressed cover many facets of climate and the climate crisis. These include the diurnal cycle; the seasons; energy differences between the continents and the oceans, the poles and the tropics; interannual variability such as Nino; natural decadal variability; and ice ages. Human-induced climate change rides on and interacts with all of these natural phenomena, and the result is an unevenly warming planet and changing weather extremes. The book emphasizes the need to not only slow or stop climate change, but also to better prepare for it and build resilience. Students, researchers, and professionals from a wide range of backgrounds will benefit from this deeper understanding of climate change.
To assess climate sensitivity from Earth radiation observations of limited duration and observed sea surface temperatures (SSTs) requires a closed and therefore global domain, equilibrium between the fields, and robust methods of dealing with noise. Noise arises from natural variability in the atmosphere and observational noise in precessing satellite observations. This paper explores the meaning of results that use only the tropical region. We compute correlations and regressions between tropical SSTs and top‐of‐atmosphere (TOA) longwave, shortwave and net radiation using a variety of methods to test robustness of results. The main changes in SSTs throughout the tropics are associated with El Niño Southern Oscillation (ENSO) events in which the dominant changes in energy into an atmospheric column come from ocean heat exchange through evaporation, latent heat release in precipitation, and redistribution of that heat through atmospheric winds. These changes can be an order of magnitude larger than the net TOA radiation changes, and their effects are teleconnected globally, and especially into the subtropics. Atmospheric model results are explored and found to be consistent with observations. From 1985 to 1999 the largest perturbation in TOA radiative fluxes was from the eruption of Mount Pinatubo and clearly models which do not include that forcing will not simulate the effects. Consequently, regressions of radiation with SSTs in the tropics may have nothing to say about climate sensitivity.
Stationary planetary waves forced by orography and diabatic beating are studied using a quasi-geostrophic two-level model on a beta-plane. This study extends a previous one by Trenberth to include the effects of a baroclinic atmosphere with zonal mean wind shear. With the introduction of vertical shear, the temperature field is no longer locked onto the heating field and can become orthogonal so that even though the wave is thermally forced there may not be any generation or loss of energy by diabatic heating. The presence of both thermal and orographic forcing together violates the conditions of nonacceleration of the zonal mean flow. The induced changes in the zonal mean flow strongly depend upon the relative phase of the thermal and orographic forcing. With a specified diabatic heating field, the eddy fluxes and changes of zonal mean flow are not sensitive to the strength of the mean zonal wind shear. By increasing the wind shear, however, the vertically propagating waves become trapped. The trapped waves accelerate the zonal mean flow through an induced meridional circulation distinct from that of the propagating waves. The characteristics of the forced waves can be explained by the local index of refraction. Results indicating important differences between the stationary planetary waves in the two hemispheres are 1) the higher total wave number in the Southern Hemisphere, which arises from the shorter meridional scale, so that the waves are trapped, and 2) interaction between thermal and orographic waves in the Northern Hemisphere which is less likely in the Southern Hemisphere. The limitations of the two-level model with the traditional fixed upper boundary condition, ω = 0 at p = 0, in reproducing forced planetary waves is evaluated by testing the sensitivity of results to a modified radiation upper boundary condition. Distinct differences are found when the planetary waves can propagate vertically. Not only is the response of the planetary waves different in phase and magnitude but also the induced acceleration of the zonal mean flow can be completely opposite. Nevertheless, there is reasonable agreement when the planetary waves are trapped vertically so that the upper boundary conditions have minor impact.
Broad vertical layer-averaged temperatures from the microwave sounder unit (MSU) are used as a quasi-independent validation of temperature fields from the U.S. National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. While the MSU and NCEP–NCAR temperatures show fairly good agreement overall, large discrepancies with ECMWF temperatures indicate that changes in the satellite observing system may have adversely affected the ECMWF reanalyses, especially in the Tropics. Two spurious discontinuities are present in tropical temperatures with jumps to warmer values throughout the Tropics below 500 mb in late 1986 and early 1989, and further spurious interannual variability is also present. These features are also reflected in the specific humidity fields. The temperature discrepancies have a complex vertical structure with height that is not fully understood, although it seems that the problems partly arise from positive reinforcement of biases in satellite radiances with those of the assimilating model first guess. Changes in the observing system provide a limit to the usefulness of the reanalyses in some climate studies.
No abstract is provided for this article.
The distribution of solar radiation absorbed on Earth is very uneven and largely determined by the geometry of the sun–earth orbit and its variations. This incoming radiant energy is transformed into various forms (internal heat, potential energy, latent energy, and kinetic energy) moved around in various ways primarily by the atmosphere and oceans; stored and sequestered in the ocean, land, and ice components of the climate system; and ultimately radiated back to space as infrared radiation. The requirement for an equilibrium climate mandates a balance between the incoming and outgoing radiation and further mandates that the flows of energy are systematic. These drive the weather systems in the atmosphere and the currents in the ocean, and they fundamentally determine the climate. They can be perturbed, causing climate change. This article examines the processes involved and follows the flows, storage, and release of energy.
An assessment is made of the global energy and hydrological cycles from eight current atmospheric reanalyses and their depiction of changes over time. A brief evaluation of the water and energy cycles in the latest version of the NCAR climate model referred to as CCSM4 is also given. The focus is on the mean ocean, land, and global precipitation P; the corresponding evaporation E; their difference corresponding to the surface freshwater flux E–P; and the vertically integrated atmospheric moisture transports. Using the model-based P and E, the time- and area-average E–P for the oceans, P–E for land, and the moisture transport from ocean to land should all be identical but are not close in most reanalyses, and often differ significantly from observational estimates of the surface return flow based on net river discharge into the oceans. Their differences reveal outstanding issues with atmospheric models and their biases, which are manifested as analysis increments in the reanalyses. The NCAR CCSM4, along with most reanalysis models, the exception being MERRA, has too-intense water cycling (P and E) over the ocean although ocean-to-land transports are very close to observed. Precipitation from reanalyses that assimilate moisture from satellite observations exhibits large changes identified with the changes in the observing system, as new and improved temperature and water vapor channels are assimilated and, while P improves after about 2002, E–P does not. Discrepancies among hydrological cycle components arise from analysis increments that can add or subtract moisture. The large-scale moisture budget divergences are more stable in time and similar across reanalyses than model-based estimates of E–P. Results are consistent with the view that recycling of moisture is too large in most models and the lifetime of moisture is too short. For the energy cycle, most reanalyses have spurious imbalances of ~10 W m−2 within the atmosphere, and ~5–10 W m−2 in net fluxes into the surface and to space. Major improvements are needed in model treatment and assimilation of moisture, and surface fluxes from reanalyses should only be used with great caution.
No abstract is provided for this article.
