Publications

Climate change is a general term for long-term changes in climate of either sign. The term “global warming” was popularized by Wally Broecker in 1975 when he published a paper in Science magazine titled “Climatic change: are we on the brink of a pronounced global warming?”; although the term had been used as early as 1957 about Roger Revelle’s research. It has most commonly been interpreted to be synonymous with rises in global mean surface temperature (GMST) and associated with increasing carbon dioxide in the atmosphere, as put forward by Broecker. However, the term is ambiguous because warming can also refer to “heating,” which is more appropriate, and just one consequence of heating is an increase in temperature (another, for example, is “melting”). In fact, this would be a better way to use the term. Another ambiguity is whether it refers to all global temperature rise, for whatever reason, or whether it refers to only anthropogenic temperature increases. In any event, many people did not like it and climate change skeptics, in particular, preferred the term “climate change” to embrace both natural and anthropogenic sources, as well as the possibility that decreases could occur.

A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.5° grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r = 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r = 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Niño– Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI < −3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI > +3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ∼20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.

The climate is changing. The main reason is because of human-induced changes in atmospheric composition which produce warming from increased greenhouse gases. This is referred to as a forcing of the climate system. There are many other forcings, both natural and anthropogenic. The issue then is to determine the consequences in terms of the change in climate and its impacts. There is a direct response to just about any forcing, and in some cases that is the answer we seek. But in many or most cases, it is not so simple. Rather, the initial change provokes other responses, especially in the atmospheric and ocean circulation, that in turn cause other changes to occur. If the response amplifies the original change, then it is referred to as a positive feedback. Whereas if the response offsets and reduces the outcome, then it is a negative feedback. The size of some effects is quantified in Section 13.5.

No abstract is provided for this article.

No abstract is provided for this article.

No abstract is provided for this article.

Major efforts are underway to improve climate models both for the advancement of science and for the benefit of society. But early results could cause problems for the public understanding of climate change.

One of the best-known and most well-respected researchers in the area of global climate change, Dr. Kevin E. Trenberth, has concluded that there can no longer be any ‘reasonable disagreement’ that human activities, and especially fossil-fuel related energy production, are causing global climate change. In a recent meeting with Refocus US Correspondent ???, Dr. Trenberth articulated his scientific judgment that the risk of not addressing climate change, through efforts such as increased emphasis on renewable energies, could be catastrophic for the planet.

No abstract is provided for this article.

An analysis and evaluation has been performed of global datasets on column-integrated water vapor (precipitable water). For years before 1996, the Ross and

A survey is made of the published estimates of the components of the poleward flux of energy by the atmosphere in the Southern Hemisphere in order to determine the total atmospheric transport. Together with recent measurements by satellite of the Earth's radiation budget this allows a new estimate of the required poleward energy transport by the oceans in the Southern Hemisphere for mean annual conditions. Results show that the ocean and atmosphere each contribute similar amounts for 0–30°S and that the ocean probably also transports about one third of the total at 60°S. The latter is in contrast to similar latitudes in the Northern Hemisphere where the ocean transport is negligible, but consistent with the different distribution of land and sea in the two hemispheres.

The tendency for more frequent El Niño events and fewer La Niña events since the late 1970's has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal‐timescale variation. One test fits an autoregressive‐moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990–1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.

Because of a lack of observations, historical simulations of land surface conditions using land surface models are needed for studying variability and changes in the continental water cycle and for providing initial conditions for seasonal climate predictions. Atmospheric forcing datasets are also needed for land surface model development. The quality of atmospheric forcing data greatly affects the ability of land surface models to realistically simulate land surface conditions. Here a carefully constructed global forcing dataset for 1948–2004 with 3-hourly and T62 (∼1.875°) resolution is described, and historical simulations using the latest version of the Community Land Model version 3.0 (CLM3) are evaluated using available observations of streamflow, continental freshwater discharge, surface runoff, and soil moisture. The forcing dataset was derived by combining observation-based analyses of monthly precipitation and surface air temperature with intramonthly variations from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis, which is shown to have spurious trends and biases in surface temperature and precipitation. Surface downward solar radiation from the reanalysis was first adjusted for variations and trends using monthly station records of cloud cover anomaly and then for mean biases using satellite observations during recent decades. Surface specific humidity from the reanalysis was adjusted using the adjusted surface air temperature and reanalysis relative humidity. Surface wind speed and air pressure were interpolated directly from the 6-hourly reanalysis data. Sensitivity experiments show that the precipitation adjustment (to the reanalysis data) leads to the largest improvement, while the temperature and radiation adjustments have only small effects. When forced by this dataset, the CLM3 reproduces many aspects of the long-term mean, annual cycle, interannual and decadal variations, and trends of streamflow for many large rivers (e.g., the Orinoco, Changjiang, Mississippi, etc.), although substantial biases exist. The simulated long-term-mean freshwater discharge into the global and individual oceans is comparable to 921 river-based observational estimates. Observed soil moisture variations over Illinois and parts of Eurasia are generally simulated well, with the dominant influence coming from precipitation. The results suggest that the CLM3 simulations are useful for climate change analysis. It is also shown that unrealistically low intensity and high frequency of precipitation, as in most model-simulated precipitation or observed time-averaged fields, result in too much evaporation and too little runoff, which leads to lower than observed river flows. This problem can be reduced by adjusting the precipitation rates using observed-precipitation frequency maps.

Nearly eight years of daily Southern Hemisphere analyses at 500 mb have been used to define the spatial dependence of the variance fields of geopotential height and the two geostrophic wind components, the corresponding covariance fields, and the transient kinetic energy. The fields are further examined in the frequency domain by using Lorenz' (1979) “poor man's spectral analysis” technique. In view of the small variation in eddy statistics as a function of the time of the year in the SH, this study removes the first four harmonies of the annual cycle and then considers all data together, so that contributions from all time scales from 2 to 4096 days (∼11 years) can be resolved. The main results are based on analyses from May 1972–January 1978 but are verified with analyses from the relatively data-rich FGGE period. Results for the zonal mean statistics are compared with those from previous studies. The zonal means of the geopotential height and westerly wind component have spectra which roughly follow that of red noise with an autocorrelation of about 0.5, whereas the northward wind component spectra closely resembles red noise with autocorretation of 0.2, resulting in considerable anisotropy in the wind fields. The northward component of transient kinetic energy is larger than the eastward component at high frequencies in middle latitudes but the reverse is true for periods of greater than two months. The westerly momentum flux by the transient eddies has a broad spectral peak at 8–32 days and is dominated by contributions from fluctuations of less than about two weeks period. The geographical dependence of the eddy statistics is mapped for four broad frequency bands covering periods of roughly less than one week, one week to two months, two months to two years, and greater than two years, thereby separating out contributions from transient baroclinic eddies, episodes of blocking, and intermonthly and interannual variability. The spatial patterns of the statistics are interpreted in the light of synoptic behavior of systems and storm tracks as defined by synoptic studies and satellite observations in the Southern Hemisphere. For periods less than a week, variances are largest in the southern Indian Ocean and relationships between the storm tracks and eddy statistics are similar to those found in the Northern Hemisphere by Blackmon, Lau, Wallace and others. However, there also are differences associated with the differences in the mean flow in each hemisphere and these are discussed in the context of baroclinic theory. At periods longer than a week geopotential height variances are largest near southern New Zealand and, to a lesser extent, southeast of South America and appear to be related to the incidence of blocking in the Southern Hemisphere. The corresponding transient kinetic energy has a maximum further north in association with cutoff cold-centered lows. In general, the high-frequency transient eddies play a much larger role in the circulation of the Southern Hemisphere than is true for the winter circulation of the Northern Hemisphere, and the eddy statistics are more zonally symmetric.

Huge heat anomalies in the atmosphere and ocean in recent years are not yet explained. Strong characteristic patterns in temperatures for upper layers of the ocean occurred from 2000 to 2023 in the presence of global warming from increasing atmospheric greenhouse gases. Here we show that the deep tropics are warming, although sharply modulated by El Niño-Southern Oscillation events, with strong heating in the extratropics near 40°N and 40-45°S, but little heating near 20°N and 25-30°S. The heating is most clearly manifested in zonal mean ocean heat content and is evident in sea surface temperatures. Strongest heating is in the Southern Hemisphere, where aerosol effects are small. Estimates are made of the contributions to heating of top-of-atmosphere (TOA) radiation, atmospheric energy transports, surface fluxes of energy, and redistribution of energy by surface winds and ocean currents. The patterns of change are not directly related to TOA radiation but are evident in net surface energy fluxes and inferred ocean heat transports, underscoring their coupled origin. Changes in the atmospheric circulation through a poleward shift in ocean jet streams and storm tracks are reflected in surface wind-driven ocean Ekman transports. As well as human-induced climate change, internal natural variability is likely in play. Hence the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates.

Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10–20°N and 18°W–20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi‐year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology 24 : 547–554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded. Copyright © 2004 Royal Meteorological Society

A review is given of the meaning of the term “El Niño” and how it has changed in time, so there is no universal single definition. This needs to be recognized for scientific uses, and precision can only be achieved if the particular definition is identified in each use to reduce the possibility of misunderstanding. For quantitative purposes, possible definitions are explored that match the El Niños identified historically after 1950, and it is suggested that an El Niño can be said to occur if 5-month running means of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceed 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas (with an equivalent definition) occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. An advantage of such a definition is that it allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or perhaps summer and peak from November to January in sea surface temperatures.