Changes In Radiative Forcing Research Articles

Sensitivity of the heat budget in a midlatitude ocean model to variations in atmospheric forcing

The rate of change of total heat storage in the ocean is determined primarily by the net heat fluxes across the air‐sea interface. In the context of a wind‐ and heat‐driven ocean model the long‐term mean heat budget was examined in a series of perturbation experiments in which atmospheric forcing parameters (surface winds and surface air temperature) were systematically varied. The thermodynamic effect of increasing surface wind speed is to take heat out of the mixed layer and thus cool the ocean. However, this wind‐induced cooling is overwhelmed by wind‐induced mechanical deepening of the mixed layer. A mixed layer deepened in this way is also cooler, so heat loss to the atmosphere is reduced. This ultimately leads to a warmer vertically integrated ocean. A change in wind stress forcing has little effect on total heat storage. Surface air temperatures are typically cooler than sea surface temperatures, so warmer air temperatures result in less heat flux out of the ocean, a warmer mixed layer, and more oceanic heat storage. The seasonal cycle is dominated by changes in radiative forcing to which the model responds exactly as it does to surface air temperature changes. As a result the model ocean stores less heat in winter (February–April) than in summer (August–October). The dependence of this seasonal change on mixed‐layer depth, mixed‐layer temperature and deep ocean temperature is quantified. Mixed‐layer depth is by far the most important controlling factor. Comparisons with climatological data from the North Atlantic and North Pacific Oceans confirm this general picture, although the comparisons suggest that the model mixed‐layer depth varies too much between seasons. The climatological data also indicate that the northern oceans store more heat in winter than in summer around 25°N. Possible explanations for this are discussed.

An Analytical Model of Atmospheric Feedback and Global Temperature Change

Abstract An analytical model of the globally averaged surface temperature response to changes in radiative forcing induced by greenhouse gases is developed from a time-dependent version of the global energy budget. The model clarifies the role of feedback and system heat capacity in controlling the magnitude and rate of response. Observed seasonal changes in surface temperature, radiative fluxes, and planetary albedo are combined to estimate the atmospheric feedback and the net gain of the system. A simple model of ocean upwelling and diffusion then yields an estimate of the heat capacity and thus the time constant of the system. The observed global temperature change from 1900 to 1990 is used to calibrate the model and refine the estimate of the time constant. The model provides a framework for comparing numerical models, including time-dependent ocean-atmosphere models used by the Intergovernmental Panel on Climate Change to estimate expected global temperature changes. When integrable analytical approx...

The influence of the albedo-temperature feed-back on climate sensitivity

A vertically integrated, zonally averaged energy-balance climate model coupled to a two-dimensional ocean model with prescribed overturning pattern is employed to assess the seasonally and latitudinally varying response of the climate system to changes in radiative forcing. Since the sensitivity of the climate system depends on its actual state, considerable attention is given to the correct simulation of the important features of the present-day climate (such as surface air temperature, sea-ice and snow amount and meridional energy transport). The climate variability induced by the various elements of the albedo-temperature (e.g. sea-ice and snow) feed-back is quantified. It appears that the variability caused by sea-ice variations is approximately twice as large as for snow variations.

The influence of the albedo-temperature feed-back on climate sensitivity

A vertically integrated, zonally averaged energy-balance climate model coupled to a two-dimensional ocean model with prescribed overturning pattern is employed to assess the seasonally and latitudinally varying response of the climate system to changes in radiative forcing. Since the sensitivity of the climate system depends on its actual state, considerable attention is given to the correct simulation of the important features of the present-day climate (such as surface air temperature, sea-ice and snow amount and meridional energy transport). The climate variability induced by the various elements of the albedo-temperature (e.g. sea-ice and snow) feed-back is quantified. It appears that the variability caused by sea-ice variations is approximately twice as large as for snow variations.

Evidence of a long‐term increase in tropospheric ozone from Pic du Midi data series: Consequences: Positive radiative forcing

The rate at which ozone is increasing in the troposphere is uncertain due to the lack of accurate long‐term measurements. Old ozone measurements obtained at the Pic du Midi Observatory (3000 m high, southwestern France) were recently rediscovered. Four sets of data available at this station are presented herein: (1) 1874–1881 and (2) 1881–1909 by the Schönbein method and (3) 1982–1984 and (4) 1990–1993 by UV absorption analyzers. The results show an increase in ozone by a factor of 5 since the beginning of the twentieth century, corresponding to an exponential increase of 1.6% per year, although this trend is probably higher (2.4% per year) for the last few decades. A stable 10 ppb ozone mixing ratio is observed during the first 20 years of the series, which is representative of the preindustrial era ozone level. The increase is seen to start around 1895. Other data, obtained at various European high‐altitude stations between 1920 and 1980, tie in closely with the Pic du Midi observations. A tentative evaluation of the impact of tropospheric ozone on radiative forcing confirms that ozone is currently the second most significant greenhouse gas, responsible for 22% and 13% of radiative forcing changes since 1800 in the northern and southern hemispheres, respectively. If these rates were to be maintained in the future, ozone would continue to evolve differently in the two hemispheres (maximum level in the northern hemisphere) and could make an even more significant contribution to the radiative forcing of the northern hemisphere.

Climate sensitivity and tropical moisture distribution

The possibility that a drying of the tropical upper troposphere might accompany tropospheric warming, substantially reducing the climate's sensitivity to a change in radiative forcing, has been subject to extensive debate. A simple one‐dimensional model of tropical convection and humidity originally proposed by Lindzen (1990a) and Sun (1990) is used to explore mechanisms for “cumulus‐induced drying,” and a narrowband radiation model to investigate its radiative implications. Low‐level moistening, which accompanies warming in all cases, significantly offsets the radiative effects of any upper‐level drying, making the net effect on radiative forcing critically sensitive to undetermined parameters in the model. When the model is tuned to reproduce mean relative humidities from Pacific radiosonde data, no substantial drying is observed to accompany a 1 K tropospheric warming. However, the clear‐sky moisture feedback is in all cases substantially less than that suggested by a scheme in which relative humidity is held constant, illustrating the importance of upper tropospheric relative humidity for climate sensitivity. Finally, the feasibility of placing constraints on critical parameters through satellite observations of interannual variability of the tropical clear‐sky radiation field is investigated.

The importance of atmospheric chemistry in the calculation of radiative forcing on the climate system

An interactive two‐dimensional model of the troposphere, stratosphere, and mesosphere, in which dynamics, radiation, and chemistry are treated interactively, is used to investigate the anthropogenic changes in the steady state chemical composition of the atmosphere since preindustrial times and to assess the associated changes in radiative forcing on climate. The perturbations in the atmospheric oxidation capacity due to anthropogenic emissions of source gases are found to be significant. In the troposphere, an ozone increase of 80–120% at northern midlatitudes and a global decrease of 10–20% in the OH concentration since the preindustrial period are calculated. In the polar lower stratosphere of the southern hemisphere, an ozone depletion since preindustrial times reaching more than 60% during spring is calculated as a result of rapid catalytical destruction of ozone by chlorine radicals in the presence of polar stratospheric clouds. Particular attention is given to the induced changes in radiative forcing. These results stress the potentially important role of chemical feedbacks on climate and indicate that the direct forcing associated with increasing concentrations of greenhouse gases is enhanced by about 30% when these feedbacks are taken into account. On a global average basis, the greenhouse effect of tropospheric ozone represents approximately 17% of the total radiative perturbation. This forcing is characterized by a strong latitudinal dependence, peaking at midlatitudes in the northern hemisphere. The importance of indirect climate forcings by stratospheric ozone (including local cooling of the stratosphere) is confirmed. It is found that the net (solar + infrared) indirect effect of stratospheric ozone changes is to increase the chlorofluorocarbon direct radiative forcing. On the other hand, the change in the longwave forcing associated with water vapor increase in the stratosphere appears to play a minor role.

A simple model for estimating methane concentration and lifetime variations

A simple methane model is presented in which lifetime changes are expressed as a function of CH4 concentration and emissions of NOx CO and NMHCs. The model parameters define the relative sensitivities of lifetime to these determining factors. The parameterized model is fitted to results from five more complex atmospheric chemistry models and to 1990 IPCC concentration projections. The IPCC data and four of the five models are well fitted, implying that the models have similar relative sensitivities. However, overall sensitivities of lifetime to changes in atmospheric composition vary widely from model to model. The parameterized model is used to estimate the history of past methane emissions, lifetime changes and OH variations, with estimates of uncertainties. The pre-industrial lifetime is estimated to be 15–34% lower than today. This implies that 23–55% of past concentration changes are due to lifetime changes. Pre-industrial emissions are found to be much higher (220–330 TgCH4/y) than the best estimate of present natural emissions (155 TgCH4/y). The change in emissions since pre-industrial times is estimated to lie in the range 160–260 TgCH4/y, compared with the current best guess for anthropogenic emissions of 360 TgCH4/y. These results imply either that current estimates of anthropogenic emissions are too high and/or that there have been large changes in natural emissions. 1992 IPCC emissions scenarios are used to give projections of future concentration and lifetime changes, together with their uncertainties. For any given emissions scenario, these uncertainties are large. In terms of future radiative forcing and global-mean temperature changes over 1990–2100 they correspond to uncertainties of at least ±0.2 Wm−2 and ± 0.1° C, respectively.

Global warming from chlorofluorocarbons and their alternatives: Time scales of chemistry and climate

The halocarbons (chloroflurocarbons, CFCs, and their replacement chemicals: the hydrochloroflurocarbons, HCFCs, and the hydrofluorocarbons, HFCs) are greenhouse gases. The atmospheric accumulation of these gases is expected to add to the global warming predicted for expected increases of CO 2, CH 4, N 2O, tropospheric ozone and H 2O. Over the next decades, production of CFCs is scheduled to be phased out, while emissions of their alternatives are expected to increase. A simple model is used to illustrate the methodology for determining the time variations of the radiative forcing and temperature changes attributable to the direct greenhouse effect from potential emissions of the halocarbons. Although there are uncertainties associated with the lifetimes of the greenhouse gases, CFCs and their substitutes, the future growth rates of these gases, and the parameters used to simulate the response of the Earth-climate system, the method serves to illustrate an important aspect of the greenhouse warming issue beyond what is provided by the various greenhouse warming indices. Our results show that for likely substitution scenarios, the warming due to halocarbons will correspond to 4–10% of the total expected greenhouse warming at the year 2100. However, uncontrolled growth of the substitutes could result in an eight-fold increase in halocarbon production and a doubling of the halocarbon contribution by 2100.

Deriving global climate sensitivity from palaeoclimate reconstructions

To assess the future impact of anthropogenic greenhouse gases on global climate, we need a reliable estimate of the sensitivity of the Earth's climate to changes in radiative forcing. Climate sensitivity is conventionally defined as the equilibrium surface temperature increase for carbon dioxide doubling, ΔT2x. Uncertainties in cloud processes spread general circulation model (GCM) estimates of this parameter over the range 1.5< ΔT2x <4.5°C (refs 1, 2). An alternative to model-based estimates is in principle available from the reconstruction of past climates3–6, which implicitly includes cloud feedback. Here we retrieve the sensitivity of two palaeoclimates, one colder and one warmer than present, by independently reconstructing both the equilibrium surface tem-perature change and the radiative forcing. Our results yield ΔT2x = 2.3 ±0.9 °C. This range is comparable with estimates from GCMs and inferences from recent temperature observations and ocean models7,8. Future application of the method to additional climates in the geological record might constrain climate sensitivity enough to narrow the model uncertainties of global warming predictions.

The Bakerian Lecture, 1991 The predictability of weather and climate

There is large public and political interest in the predictability of weather and climate, in particular in the influence of human activities on the likely climate change during the next century. Numerical models are the main tools which enable the nonlinear processes involved in the dynamics and physics of the atmosphere and other components of the climate system to be integrated in an effective way. The performance of such models used for weather forecasting has continued to improve as more accurate data with better coverage has become available, as improved descriptions of the physics and dynamics have been incorporated and as computing capacity and speed have increased. Studies of the predictability with models suggest that with further improvements in data and models deterministic forecasting of detailed weather may ultimately have useful skill up to 2-3 weeks ahead. Beyond the limit of deterministic forecasting, some skill remains for the forecasting of general weather patterns which can be pursued by studying ensembles of model forecasts from slightly varying initial conditions. The largest difficulty with further improvements of numerical models lies in their inadequate treatment of the motions too small to be explicitly resolved. Interactions between the atmosphere and the ocean are responsible for substantial variations on seasonal, interannual and longer timescales. Forecasts are being provided of seasonal precipitation in the Sahel region of Africa based on a knowledge of global sea surface tem perature (SST) anomalies together with the assumption that such anomalies tend to persist from one season to the next. Attempts to forecast SST anomalies have centred on tropical regions in particular on the El Nino. Simple models show some skill in forecasting El Nino events 3-9 months in advance. Studies with more elaborate models which as yet only show partial success in simulating these events demonstrate the complex nature of the interactions involved. Turning to the likely changes in climate next century: if no changes occur in the atmosphere other than the increase in C0 2 and other greenhouse gases due to human activities, the increase in radiative forcing due to a doubling of atmospheric C0 2 concentration would lead to an increase of about 1.2 °C in global average temperature. Water vapour and ice-albedo feedbacks raise this to a figure of about 2.5 °C (with an uncertainty range of 1.5—4.5 °C) as estimated by the Intergovernmental Panel for Climate Change. Such a change would dominate over forcing likely to arise from other factors, and this estim ated rate of change next century is probably greater than any which has occurred on earth during the past 10000 years. The main uncertainties in climate change predictions arise from the inadequacies of the models in their descriptions of cloud-radiation and ocean circulation feedbacks. Until there is more confidence in the treatment of these feedbacks there are bound to be large uncertainties associated with any predictions of regional climate change. To reduce the uncertainties there need to be improvements in computer power, in model formulation and in our understanding of climate processes together with a large programme of observations of climate parameters to provide early detection of climate change and to provide validation of climate models and to provide data for initialization of model integrations. An important question is whether changes in climate due to changes in radiative forcing are predictable. It is pointed out that the response to climate over the past half million years to changes in forcing due to the variations in the Earth ’s orbit (Milankovitch cycles) is a regular one; some 60% of variations in the global temperature as established from the palaeontological record occur near frequencies of the Milankovitch cycles. We can, therefore, expect the changes in climate due to increasing greenhouse gases to be a largely predictable response. Large, but probably predictable, changes in the circulation of the deep ocean have modified climate change during past epochs and could have significant influence on future climate change.

Climate Change Scenarios for the UK

The purpose of this paper is to present time-dependent scenarios of climate change for the UK, including possible changes in extreme climatic events. Initially, the methods of scenario construction are described. These involve the scaling of the regional patterns of equilibrium climate change produced by general circulation models (GCMs) by the time-dependent projection of global-mean temperature change. A 'Business-as-Usual' forcing scenario is then used to project global warming to the year 2050. This projection is used to develop scenarios of the UK temperature and precipitation change. The implications for sea level are also discussed. Finally, the climate change 'commitment' for the UK, due to past radiative forcing changes and the lags in the climate system, is considered.

Interpretation of Short-Term Solar Variability Effects in the Troposphere

Among possible external forcing agents for tropospheric response to solar variability, the variation of MeV-GeV particle fluxes, resulting in changes in stratosphere-troposphere ionization and electric fields, is a plausible candidate. The MeV-GeV particle hypothesis fits reported correlations of geopotential heights, temperatures and vorticity area index (VAT) on the one hand with solar proton events, high-speed solar wind plasma streams, magnetic storms and Forbush decreases of galactic cosmic rays on the other hand. Both solar wind magnetic structures and changes in the terrestrial magnetosphere modulate the incoming particle fluxes. The day-to-day correlations are strongest in winter.Mechanisms linking MeV-GeV flux changes with tropospheric changes are speculative. Among a number of processes to be investigated, one not previously considered involves the electrofreezing of supercooled water droplets in high clouds of cyclones; their growth by the Wegener-Bergeron instability; sedimentation to lower level supercooled clouds where they enhance freezing and latent heat release; the intensification of convection; and in warm core winter cyclones the extraction of energy from the baroclinic instability to further intensify the cyclone; leading to changes in the general circulation. On a longer time scale changes in water/ice ratio in cirrus may lead to changes in cloud radiative forcing and also to changes in the general circulation.

Solar variability influences on weather and climate: Possible connections through cosmic ray fluxes and storm intensification

The question of the mechanism for solar‐variability effects on weather and climate can be separated into (1) the identification of the carrier of the solar variability and (2) the identification of the physical link between the carrier and the meteorological response. The suggestion that galactic cosmic rays (GCR), as modulated by the solar wind, are the carriers of the component of solar variability that affects weather and climate has been discussed in the literature for 30 years, and considerable evidence for it has now accumulated. Variations of GCR occur with the 11‐year solar cycle, matching the time scale of recent results for atmospheric variations, as modulated by the quasi‐biennial oscillation of equatorial stratospheric winds (QBO). Variations in GCR occur on the time scale of centuries with a well‐defined peak in the coldest decade of the little ice age. Here we present new evidence on the meteorological responses to variations on the time scale of a few days. The occurrence of correlations of GCR and meteorological responses on all three time scales strengthens the hypothesis of GCR as carriers of solar variability to the lower atmosphere. The responses reported here include changes in the vertical temperature profile in the troposphere and lower stratosphere and in the northern hemisphere vorticity area index, associated with Forbush decreases in GCR. The meteorological responses to Forbush decreases are in the opposite sense but otherwise are quite similar to responses that immediately follow solar flares. This is to be expected, based on the hypothesis that particles with energy about 100–1000 MeV are the external forcing function for the tropospheric response, since large solar flares increase the particle flux and ionization and minor species production in the lower stratosphere, whereas Forbush decreases reduce them. The mechanism or mechanisms linking changes in low‐energy GCR and other particles in this energy range of 100–1000 MeV to tropospheric temperature and dynamic responses have not been identified. This can be attributed to current uncertainties regarding the microphysical and electrical properties of aerosols and clouds. One possibility is that changes in clouds lead to changes in cloud radiative forcing. The height distribution of the tropospheric response and the amount of energy involved and the rapidity of the time response suggest that the release of latent heat could also be involved. These could lead to the observed tropospheric responses which are understandable in terms of changes in the intensity of cyclonic disturbances. Theoretical considerations link such changes to the observed latitudinal movement of the jet stream.

On the role of high latitude ice, snow, and vegetation feedbacks in the climatic response to external forcing changes

A seasonal energy balance climate model containing a detailed treatment of surface and planetary albedo, and in which seasonally varying land snow and sea ice amounts are simulated in terms of a number of explicit physical processes, is used to investigate the role of high latitude ice, snow, and vegetation feedback processes. Feedback processes are quantified by computing changes in radiative forcing and feedback factors associated with individual processes. Global sea ice albedo feedback is 5–8 times stronger than global land snowcover albedo feedback for a 2% solar constant increase or decrease, with Southern Hemisphere cryosphere feedback being 2–5 times stronger than Northern Hemisphere cryosphere feedback.