519 publications from this institution
Changes in the average net radiation at the top of the atmosphere due to perturbations in the incident solar radiation from the changes internal to the Sun or from changes in the orbit of Earth around the Sun lead to a change in heating, and climate change. There is no doubt about this physically, but the question is how large are these fluctuations?
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A detailed examination is made in both observations and the Community Earth System Model (CESM) of relationships among top‐of‐atmosphere radiation, water vapor, temperatures, and precipitation for 2000–2014 to assess the origins of radiative perturbations and climate feedbacks empirically. The 30‐member large ensemble coupled runs are analyzed along with one run with specified sea surface temperatures for 1994 to 2005 (to avoid volcanic eruptions). The vertical structure of the CESM temperature profile tends to be top heavy in the model, with too much deep convection and not enough lower stratospheric cooling as part of the response to tropospheric heating. There is too much absorbed solar radiation (ASR) over the Southern Oceans and not enough in the tropics, and El Niño–Southern Oscillation (ENSO) is too large in amplitude in this version of the model. However, the covariability of monthly mean anomalies produces remarkably good replication of most of the observed relationships. There is a lot more high‐frequency variability in radiative fluxes than in temperature, highlighting the role of clouds and transient weather systems in the radiation statistics. Over the Warm Pool in the tropical western Pacific and Indian Oceans, where nonlocal effects from the Walker circulation driven by the ENSO events are important, several related biases emerge: in response to high SST anomalies there is more precipitation, water vapor, and cloud and less ASR and outgoing longwave radiation in the model than observed. Different model global mean trends are evident, however, possibly hinting at too much positive cloud feedback in the model.
The answers I frequently get to the question “What is climate?” are commonly along the lines of “the average weather” or “climate is what we expect and weather is what we get.” Firstly, those are statistical statements, and secondly, an average is obviously dependent upon the time of the average. If it is a very “long-term” average to avoid interannual variability, then by definition there is no climate change. This conundrum was recognized back in the 1970s when it was proposed that we speak about “climate states.” This perhaps relates to what is now commonly known as a “base period.” The classic base period is a 30-year period (as defined by the World Meteorological Organization) that traditionally gets updated. Hence we went from the 1961–1990 normal to the 1971-2000 normal, and now 1981–2010 is the “New Normal.”
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An analysis has been made of the means and variability of the 500 mb field in the Southern Hemisphere, with accent on the zonal means of geopotential height and westerly wind. Long-term means for May 1972-January 1978 are significantly different from previous analyses and reveal very large and significant trends in geopotential height, especially over Antarctica. Standard deviations of zonal winds and heights are larger than in the Northern Hemisphere and, in contrast to the Northern Hemisphere, are lowest in winter. Temporal variations in the zonal mean component of the flow are very pronounced and some aspects of the very anomalous flow in December 1976 are documented. Interannual variations show a remarkable quasi-biennial fluctuation in the zonal mean fields with a systematic progression of the anomalies from low to high latitudes. These are correlated with the quasi-biennial oscillation in the equatorial stratosphere.
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The diurnal range of surface air temperature (DTR) has decreased worldwide during the last 4–5 decades and changes in cloud cover are often cited as one of the likely causes. To determine how clouds and moisture affect DTR physically on daily bases, the authors analyze the 30-min averaged data of surface meteorological variables and energy fluxes from the the First International Satellite Land Surface Climatology Project Field Experiment and the synoptic weather reports of 1980–1991 from about 6500 stations worldwide. The statistical relationships are also examined more thoroughly in the historical monthly records of DTR, cloud cover, precipitation, and streamflow of this century. It is found that clouds, combined with secondary damping effects from soil moisture and precipitation, can reduce DTR by 25%–50% compared with clear-sky days over most land areas; while atmospheric water vapor increases both nighttime and daytime temperatures and has small effects on DTR. Clouds, which largely determine the geographic patterns of DTR, greatly reduce DTR by sharply decreasing surface solar radiation while soil moisture decreases DTR by increasing daytime surface evaporative cooling. Clouds with low bases are most efficient in reducing the daytime maximum temperature and DTR mainly because they are very effective in reflecting the sunlight, while middle and high clouds have only moderate damping effects on DTR. The DTR reduction by clouds is largest in warm and dry seasons such as autumn over northern midlatitudes when latent heat release is limited by the soil moisture content. The net effects of clouds on the nighttime minimum temperature is small except in the winter high latitudes where the greenhouse warming effect of clouds exceeds their solar cooling effect. The historical records of DTR of the twentieth century covary inversely with cloud cover and precipitation on interannual to multidecadal timescales over the United States, Australia, midlatitude Canada, and former U.S.S.R., and up to 80% of the DTR variance can be explained by the cloud and precipitation records. Given the strong damping effect of clouds on the daytime maximum temperature and DTR, the well-established worldwide asymmetric trends of the daytime and nighttime temperatures and the DTR decreases during the last 4–5 decades are consistent with the reported increasing trends in cloud cover and precipitation over many land areas and support the notion that the hydrologic cycle has intensified.
Temperatures on Earth are rising at the surface and especially for the oceans. But how is it known that this is due to human activities? The main way this question has been addressed in most places is to use climate modeling, where a climate model can be run with and without various forcings and the results compared with observations. In these models, the increase in carbon dioxide is generally assumed. But how is it known that the increased carbon dioxide comes from human activities?
An outline is given of the IPCC and its role in helping to lay out options and consequences of actions or lack thereof related to anthropogenic climate change. A description is provided of how the IPCC is set up and functions, the scientific assessment process, who participates, and the intergovernmental linkages, all of which lead to the IPCC reports every 5–7 years. The procedures are designed to provide policy relevant but not policy prescriptive scientific advice to policy makers and the public. The main findings are briefly described along with the recent controversy involving IPCC.
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. Greenhouse-gas emissions have created a planetary energy imbalance that is primarily manifested by increasing ocean heat content (OHC). Updated observational estimates of full-depth OHC change since 1970 are presented that account for recent advancements in reducing observation errors and biases. The full-depth OHC has increased by 0.74 [0.68, 0.80] × 1022 J/yr (0.46 Wm−2) and 1.22 [1.16–1.29] ×1022 J/yr (0.75 Wm−2) for 1970-2005 and 1992-2005 respectively, with 5 % to 95 % confidence interval of the median. The CMIP5 models show large spread in OHC changes, but the ensemble median has excellent agreement with our observational estimate: 0.68 [0.54–0.82] × 1022 J/yr (0.41 Wm−2) from 1970 to 2005 and 1.25 [1.10–1.41] × 1022 J/yr (0.77 Wm−2) from 1992 to 2005. These results increase confidence in both the observational and model estimates to quantify and study changes in Earth’s energy imbalance over the historical period. We suggest that OHC be a fundamental metric for climate model validation and evaluation.
As observations and atmospheric reanalyses have improved, the diagnostics that can be computed with confidence also increase. Accordingly, a new formulation of the energetics of the atmosphere is laid out, with a view to advancing diagnostic studies of Earth’s energy budget and flows. It is utilized to produce assessments of the vertically integrated divergences in both the atmosphere and ocean. Careful conservation of mass is required, with special attention given to the hydrological cycle and redistribution of mass associated with precipitation and evaporation, and a new method for ensuring this is developed. It guarantees that the atmospheric divergence is associated with moisture and precipitation, unlike previous methods. A new term, identified as associated with the enthalpy of precipitation, is included in a preliminary way. It is sensitive to the formulation, and the use of temperature in degrees Celsius instead of Kelvin greatly reduces errors and produces the extra term with values up to about ±5 W m −2 . New results for 2000 to 2016 are presented for the vertical-mean and annual-mean diabatic atmospheric heating, atmospheric moistening, and total atmospheric energy divergence. Results for the atmospheric divergence are combined with top-of-atmosphere radiation observations to deduce total surface energy fluxes. Along with estimates of changes in ocean heat content, the Atlantic Ocean meridional heat transports are recomputed for March 2000 through 2013. The new results are compared with previous estimates and an assessment is made of the effects of the new mass balance, change in temperature scale, and the extra precipitation enthalpy term.
[1] The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichon in March 1982 and Pinatubo in June 1991, taking into account changes from El Nino-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991–September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.
The zonally asymmetric component of the 500 mb flow in the Southern Hemisphere has been analyzed into wavenumber space, and the annual cycle and interannual variability are documented for the period May 1972–January 1978. The planetary waves 1–3 account for most of the variance and all three waves make important contributions, especially to the interannual variability, although the mean fields are dominated by wave 1. The seasonal variation in all fields is small compared with that in the Northern Hemisphere. In the subtropics, the interannual fluctuations in the planetary waves are mainly coupled to the Southern Oscillation and the wave fluctuations are coupled to those in the mean zonal westerlies.
