Climate observations are needed to address a large range of important societal issues including sea level rise, droughts, floods, extreme heat events, food security, and freshwater availability in the coming decades. Past, targeted investments in specific climate questions have resulted in tremendous improvements in issues important to human health, security, and infrastructure. However, the current climate observing system was not planned in a comprehensive, focused manner required to adequately address the full range of climate needs. A potential approach to planning the observing system of the future is presented in this article. First, this article proposes that priority be given to the most critical needs as identified within the World Climate Research Program as Grand Challenges. These currently include seven important topics: melting ice and global consequences; clouds, circulation and climate sensitivity; carbon feedbacks in the climate system; understanding and predicting weather and climate extremes; water for the food baskets of the world; regional sea‐level change and coastal impacts; and near‐term climate prediction. For each Grand Challenge, observations are needed for long‐term monitoring, process studies and forecasting capabilities. Second, objective evaluations of proposed observing systems, including satellites, ground‐based and in situ observations as well as potentially new, unidentified observational approaches, can quantify the ability to address these climate priorities. And third, investments in effective climate observations will be economically important as they will offer a magnified return on investment that justifies a far greater development of observations to serve society's needs.
No abstract is provided for this article.
Radiation is a fundamental variable for the whole flow of energy, and variations are perceived mainly by changes in cloudiness. Accordingly, the incoming and outgoing radiation are included here.
From 1979 to 1989, global European Centre for Medium Range Weather Forecasts (ECMWF) analyses of many meteorological variables important in baroclinic storms have been bandpass filtered to highlight fluctuations on 2- to 8-day time scales in order to illuminate the relationships among the variables in baroclinic storm-track regions. Because of the greater zonal symmetry in the Southern Hemisphere (SH) versus the Northern Hemisphere (NH), it is possible to examine these relationships over a zonal sector without being dominated by local effects associated with jet stream entrance and exit regions of the storm tracks. The bandpassed variance of geopotential height is used to define the storm track, and its meridional profile has a pronounced midlatitude maximum in the SH. The locations of the jet stream and the variance maxima of meridional and zonal velocity components, vorticity, vertical motion, specific humidity, and temperature all bear a distinctive relationship to the storm track, but the latitudes of their maxima are displaced from the center of the storm track. Many covariance quantities such as the transient eddy temperature, moisture, vorticity, and vertical and horizontal momentum fluxes also exhibit a strong storm-track signature. The observed relationships among the eddy quantities can generally be understood in terms of geostrophic theory and perturbation analysis applied to baroclinic systems. Storm-track activity is remarkably persistent throughout the year in both location and intensity in the SH. The storm track is farthest poleward in the transition seasons as part of a semiannual cycle, but remains near 50°S year round, and is strongest in the southern Indian Ocean and weakest in the South Pacific. There is a strong relationship between the storm track and the major tropospheric polar jet stream and its associated lower-level baroclinicity throughout the year, and the distribution of storm-track activity can be accounted for by baroclinic theory. In contrast to the NH, strongest meridional temperature gradients in middle latitudes are found in the summer half-year in the SH. Whereas the NH storm-track activity is much weaker in summer and shifts poleward, the SH activity is as strong as in winter and, if anything, shifts slightly equatorward. The zonal symmetry is greater in summer and meridional profiles are sharper, implying less variability in the storm track both within and between seasons. In winter, high-frequency storm-track activity extends over a broader range of latitudes and continues to be mainly associated with the polar jet stream. The question of the impact of the storm-track eddies on the mean flow is examined using zonal mean and a localized Eliassen–Palm flux. Baroclinic effects from the poleward heat flux dominate in winter and the eddies act to decelerate the upper tropospheric westerlies. Barotropic effects in the upper troposphere, mostly from the meridional momentum flux convergence, help to maintain the mean westerly distribution by accelerating the main polar jet and maintaining the mean split in the flow near New Zealand in both summer and winter and even dominate the baroclinic component in the storm track in summer. Diabatic heating by the eddies may also help maintain the mean flow.
Modern climate change is dominated by human influences, which are now large enough to exceed the bounds of natural variability. The main source of global climate change is human-induced changes in atmospheric composition. These perturbations primarily result from emissions associated with energy use, but on local and regional scales, urbanization and land use changes are also important. Although there has been progress in monitoring and understanding climate change, there remain many scientific, technical, and institutional impediments to precisely planning for, adapting to, and mitigating the effects of climate change. There is still considerable uncertainty about the rates of change that can be expected, but it is clear that these changes will be increasingly manifested in important and tangible ways, such as changes in extremes of temperature and precipitation, decreases in seasonal and perennial snow and ice extent, and sea level rise. Anthropogenic climate change is now likely to continue for many centuries. We are venturing into the unknown with climate, and its associated impacts could be quite disruptive.
No abstract is provided for this article.
A perspective is given on human-induced climate change, contrasting two complementary approaches. The first is the conventional approach of using climate models as a means for developing understanding of the climate system variations and for projections of the future. Climate models have improved enormously, but even with the biggest supercomputers, models cannot resolve scales needed to depict many important phenomena. Major challenges remain in addressing chaotic natural weather and climate variability, and processing huge volumes of data. The second is an approach based upon understanding the changing Earth's energy imbalance (EEI) and tracking the consequences through the flows of energy through the climate system. A new assessment is given of the EEI through estimates of the uptake of heat by glaciers and ice sheets, land, and the oceans, and implications for the hydrological cycle. The oceans take up 93% of EEI of 0.9 ± 0.2 W m−2 (or 430 TW). Climate change is already readily apparent, with major consequences and costs.
The global physical and biogeochemical environment has been substantially altered in response to increased atmospheric greenhouse gases from human activities. In 2023, the sea surface temperature (SST) and upper 2000 m ocean heat content (OHC) reached record highs. The 0–2000 m OHC in 2023 exceeded that of 2022 by 15 ± 10 ZJ (1 Zetta Joules = 10 21 Joules) (updated IAP/CAS data); 9 ± 5 ZJ (NCEI/NOAA data). The Tropical Atlantic Ocean, the Mediterranean Sea, and southern oceans recorded their highest OHC observed since the 1950s. Associated with the onset of a strong El Niño, the global SST reached its record high in 2023 with an annual mean of ∼0.23°C higher than 2022 and an astounding > 0.3°C above 2022 values for the second half of 2023. The density stratification and spatial temperature inhomogeneity indexes reached their highest values in 2023.
No abstract is provided for this article.
A comprehensive comparison is made between two tropospheric temperature datasets over the period 1979–98: the most recent and substantially revised (version d) microwave sounding unit (MSU) channel 2 data retrievals, and a gridded radiosonde analysis provided by the Hadley Centre of the U.K. Meteorological Office. The latter is vertically weighted to approximate the deep layer temperatures measured by the satellite data. At individual grid points, there is good overall agreement among monthly anomalies, especially over the Northern Hemisphere continents where the climate signal is large, although monthly root-mean-square (rms) differences typically exceed 0.6°C. Over the Tropics, correlations are lower and rms differences can be as large as the standard deviations of monthly anomalies. Differences in the gridpoint variances are significant at many locations, which presumably reflects sources of noise in one or both measurement systems. It is often argued for climate purposes that temperature anomalies are large in scale so that averaging over larger areas better serves to define the anomalies while reducing sampling error. This is the case for the Tropics (20°S–20°N) where the large signal associated with El Niño-Southern Oscillation events is well captured in both datasets. Over the extratropics, however, the results indicate that it is essential to subsample the satellite data with the radiosonde coverage in both space and time in any evaluation. For collocated global average monthly anomalies, correlations are ~0.9 with rms differences ~0.10°C for both lower- (MSU2LT) and mid- (MSU2) tropospheric anomalies. The agreement between the satellite and radiosonde data is slightly better for the latest version of MSU2LT than it is for MSU2, in spite of the higher noise levels of the former. This is primarily attributable to a strong warming trend in the MSU2, data relative to the radiosonde data toward the end of the record. Given the global nature of this discrepancy, it is suspected that it primarily reflects problems in the MSU analysis. As radiosonde records almost universally contain temporal inhomogeneities as well, caution is required when interpreting trends, which are not known to within 0.1 °C decade−1. However, the evidence suggests that global surface air temperatures are indeed warming at a significantly faster rate than tropospheric temperatures over the past 20 yr, and this is primarily attributable to physical differences in these two quantities.
Ocean meridional heat transports (MHTs) are deduced as a residual using energy budgets to produce latitude versus time series for the globe, Indo-Pacific, and Atlantic. The top-of-atmosphere (TOA) radiation is combined with the vertically integrated atmospheric energy divergence from atmospheric reanalyses to produce the net surface energy fluxes everywhere. The latter is then combined with estimates of the vertically integrated ocean heat content (OHC) tendency to produce estimates of the ocean heat divergence. Because seasonal sea ice and land runoff effects are not fully considered, the mean annual cycle is incomplete, but those effects are small for interannual variability. However, there is a mismatch between 12-month inferred surface flux and the corresponding OHC changes globally, requiring adjustments to account for the Earth’s global energy imbalance. Estimates are greatly improved by building in the constraint that MHT must go to zero at the northern and southern extents of the ocean basin at all times, enabling biases between the TOA and OHC data to be reconciled. Zonal mean global, Indo-Pacific, and Atlantic basin ocean MHTs are computed and presented as 12-month running means and for the mean annual cycle for 2000–16. For the Indo-Pacific, the tropical and subtropical MHTs feature a strong relationship with El Niño–Southern Oscillation (ENSO), and in the Atlantic, MHT interannual variability is significantly affected by and likely influences the North Atlantic Oscillation (NAO). However, Atlantic and Pacific changes are linked, suggesting that the northern annular mode (as opposed to NAO) is predominant. There is also evidence of decadal variability or trends.
A review is given of the trends, variability, mean and annual cycle of energy flowing through the climate system, and its storage, release, and transport in the atmosphere, ocean, and land surface as estimated with recent observations, with some new updates using the latest datasets. The current imbalance in radiation at the top-of-atmosphere owing to human-induced increases in greenhouse gases means that the atmosphere, land and ocean are warming up, and ice is melting, leading to a rise in sea level. A discussion is given of our ability to track these changes with current observations and analyses. Current global analyses of the atmosphere and the ocean contain spurious variability on decadal time scales that arises from inadequacies and changes in the observing system. A holistic integrated approach that brings all information to bear can provide constraints on what is happening and where the main weaknesses are in the observing system. Results for ocean heat content are discussed in the light of recent corrections to ocean soundings and new ocean observations, and in the context of the thermosteric contributions to sea level rise.
Climate projections point toward more frequent and intense weather and climate extremes such as heat waves, droughts, and floods, in a warmer climate. These projections, together with recent extreme climate events, including flooding in Pakistan and the heat wave and wildfires in Russia, highlight the need for improved risk assessments to help decision makers and the public. But accurate analysis and prediction of risk of extreme climate events require new methodologies and information from diverse disciplines. A recent workshop sponsored by the World Climate Research Programme (WCRP) and hosted at United Nations Educational, Scientific and Cultural Organization (UNESCO) headquarters in France brought together, for the first time, a unique mix of climatologists, statisticians, meteorologists, oceanographers, social scientists, and risk managers (such as those from insurance companies) who sought ways to improve scientists' ability to characterize and predict climate extremes in a changing climate.
The total mass of the atmosphere varies mainly from changes in water vapor loading; the former is proportional to global mean surface pressure and the water vapor component is computed directly from specific humidity and precipitable water using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analyses (ERA-40). Their difference, the mass of the dry atmosphere, is estimated to be constant for the equivalent surface pressure to within 0.01 hPa based on changes in atmospheric composition. Global reanalyses satisfy this constraint for monthly means for 1979–2001 with a standard deviation of 0.065 hPa. New estimates of the total mass of the atmosphere and its dry component, and their corresponding surface pressures, are larger than previous estimates owing to new topography of the earth’s surface that is 5.5 m lower for the global mean. Global mean total surface pressure is 985.50 hPa, 0.9 hPa higher than previous best estimates. The total mean mass of the atmosphere is 5.1480 × 1018 kg with an annual range due to water vapor of 1.2 or 1.5 × 1015 kg depending on whether surface pressure or water vapor data are used; this is somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27 × 1016 kg and the dry air mass as 5.1352 ± 0.0003 × 1018 kg. The water vapor contribution varies with an annual cycle of 0.29-hPa, a maximum in July of 2.62 hPa, and a minimum in December of 2.33 hPa, although the total global surface pressure has a slightly smaller range. During the 1982/83 and 1997/98 El Niño events, water vapor amounts and thus total mass increased by about 0.1 hPa in surface pressure or 0.5 × 1015 kg for several months. Some evidence exists for slight decreases following the Mount Pinatubo eruption in 1991 and also for upward trends associated with increasing global mean temperatures, but uncertainties due to the changing observing system compromise the evidence. The physical constraint of conservation of dry air mass is violated in the reanalyses with increasing magnitude prior to the assimilation of satellite data in both ERA-40 and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses. The problem areas are shown to occur especially over the Southern Oceans. Substantial spurious changes are also found in surface pressures due to water vapor, especially in the Tropics and subtropics prior to 1979.
It has been traditional in meteorology to divide the velocity field up into rotational and divergent components, but not the geopotential field. Yet any balance condition, such as the geostrophic relation or linear balance equation, is a diagnostic relation which states that not only can the balanced velocity field be computed from the geopotential field, but also that the geopotential can be derived from the velocity field. If the latter approach is adopted, then the difference between the observed and computed geopotential is the quantity we refer to as the divergent, or in some cases, ageostrophic, geopotential. In fact, for any balance set of equations, it is essential to partition the geopotential in such a way in order to derive an equivalent set of momentum equations. The momentum equations equivalent to the linear balance set of equations are given. The linear balance equation is integrated to give a diagnostic relation between the rotational wind components and the gradient of the rotational geopotential plus an extra term which involves the gradient of the planetary vorticity advection potential (PVAP). The partitioning of the velocity field into rotational and divergent parts v = vr + vd does not depend on any other field. Given vr and the associated streamfunction, we have computed Φ, from the linear balance equation. The difference Φd = Φ − Φr is of order Rossby number times Φ, has global scale dominated by wave 1, and tends to be a maximum near the equator. This partitioning depends upon the balance equation used. The different components of Φ have implications for how the observed Φ should and should not be used in diagnostic studies and provide a new interpretation as to what the ageostrophic component of the flow is. In particular, the ageostrophic wind is partitioned into the divergent wind plus contributions arising from the gradient of the PVAP and the gradient of the divergent geopotential rotated 90°. The rotational and divergent geopotential fields and the PVAP have been computed from climatological mean January and July conditions. In addition for January, the tendencies due to the Coriolis form associated with the ageostrophic velocity are given and are shown to be related to the acceleration of jets in entrance regions and deceleration in exit regions of in excess of 30 m s−1/day for the Northern Hemisphere. However, while all three terms contributing to the ageostrophic velocity are of roughly equal importance overall, the divergent wind is less important in jet entrance and exit regions and the gradients of the PVAP and Φd terms are dominant. This illustrates the kinematic nature of these acceleration terms and shows the balance that exists in the momentum budget, but provides little insight into the cause of the existence of the mean jets.
Analysis of global surface air temperature records has indicated that recent years have been among the warmest since the late nineteenth century1, with 1995 being the warmest year on record2. But the rate of global annual mean surface warming of 0.13 °C per decade during the period 1979–95 differs substantially from the global lower-tropospheric cooling trend of – 0.05 °C per decade3 Inferred from the record (MSU-2R) of radiance measurements by the satellite Microwave Sounder Unit (MSU)4,5. Accordingly, the satellite record has been widely cited by sceptics as evidence against global warming6–10. However, a substantial fraction of the measured radiance originates not from the atmosphere but from the Earth's surface11, and gives rise to high noise levels. This noise can lead to errors when merging temperature time series obtained from different satellites. Here we present comparisons among different MSU retrievals, sea surface temperatures (SSTs), and equivalent MSU temperatures derived from an atmospheric general circulation model forced with observed SSTs. The comparisons, focused on the tropics where atmospheric temperatures are closely tied to SSTs, strongly suggest that two spurious downward jumps occur in the MSU-2R record coinciding with changes in satellites, and that the real trend in MSU temperatures is likely to be positive, albeit small.
The co‐variability of monthly mean surface temperature and precipitation is determined globally for 1979–2002 from observationally‐based analyses (ERA‐40) for surface air temperature and the Global Precipitation Climatology Project (GPCP) version 2 for precipitation and compared with results from the NCAR Community Atmospheric Model version 3 (CAM3) and Community Climate System Model version 3 (CCSM3). Results are combined for the 5 months for northern winter (November to March) and summer (May to September). Over land, negative correlations dominate, as dry conditions favor more sunshine and less evaporative cooling, while wet summers are cool. At high latitudes in winter, positive correlations dominate as warm moist advection in extratropical cyclones favors precipitation and the water holding capacity of the atmosphere limits precipitation amounts in cold conditions. Where ocean conditions drive the atmosphere, higher surface air temperatures are associated with precipitation, as in El Niño, but some areas, such as the western Pacific in northern summer, feature negative correlations indicating that the atmosphere determines the surface temperatures. In the CAM driven with observed sea surface temperatures and the CCSM in fully coupled mode the latter mechanism is largely absent, and correlations are generally much stronger than observed, indicating more local control. Neither temperature nor precipitation records should be interpreted without considering the strong covariability that exists.
