Climate response times: dependence on climate sensitivity and ocean mixing

CODC-v1: a quality-controlled and bias-corrected ocean temperature profile database from 1940-2023

High-quality ocean in situ profile observations are fundamental for ocean and climate research and operational oceanographic applications. Here we describe a new global ocean subsurface temperature profile database named the Chinese Academy of Science (CAS) Oceanography Data Center version 1 (CODC-v1). This database contains over 17 million temperature profiles between 1940-2023 from all available instruments. The major data source is the World Ocean Database (WOD), but CODC-v1 also includes some data from some Chinese institutes which are not available in WOD. The data are quality-controlled (QC-ed) by a new QC system that considers the skewness of local temperature distributions, topographic barriers, and the shift of temperature distributions due to climate change. Biases in Mechanical Bathythermographs (MBTs), eXpendable Bathythermographs (XBTs), and Bottle data (OSD) are all corrected using recently proposed correction schemes, which makes CODC-v1 a bias-corrected dataset. These aspects ensure the data quality of the CODC-v1 database, making it suitable for a wide spectrum of ocean and climate research and applications.

Common Issues in Verification of Climate Forecasts and Projections

With increased interest in climate forecasts and projections, it is important to understand more about their sources and levels of skill. A starting point here is to describe the nature of the skill associated with forecasts and projections. Climate forecasts and projections typically both include time varying forcing of the climate, but only forecasts have initial conditions set close to the observed climate state. Climate forecasts therefore derive skill from both initial conditions and from forcing. The character of the initial condition skill and forcing skill is different. Skill from initial conditions results in a narrowing of expectations relative to a climatological distribution and points toward a more favoured part of the distribution. Forcing skill could result from a shift in the preferred parts of the climatological distribution in response to forcing, or it could result from a shift in the entire distribution, or both. Assessments of forcing skill require time averages of the target variable that are long enough so that the contributions from internal variations are small compared to the forced response. The assessment of skill of climate forecasts and projections is inherently partial because of the small number of repeated trials possible on typical climate time scales but is nonetheless the only direct measure of their performance.

Asymmetrical response of summer rainfall in East Asia to CO2 forcing

Understanding the regional hydrological response to varying CO₂ concentration is critical for cost-benefit analysis of mitigation and adaptation polices in the near future. To characterize summer monsoon rainfall change in East Asia in a changing CO₂ pathway, we used the Community Earth System Model (CESM) with 28 ensemble members in which the CO₂ concentration increases at a rate of 1% per year until its quadrupling peak, i.e., 1468 ppm (ramp-up period), followed by a decrease of 1% per year until the present-day climate conditions, i.e., 367 ppm (ramp-down period). Although the CO₂ concentration change is symmetric in time, the amount of summer rainfall anomaly in East Asia is increased 42% during a ramp-down period than that during a ramp-up period when the two periods of the same CO₂ concentration are compared. This asymmetrical rainfall response is mainly due to an enhanced El Niño-like warming pattern as well as its associated increase in the sea surface temperature in the western North Pacific during a ramp-down period. These sea surface temperature patterns enhance the atmospheric teleconnections and the local meridional circulations around East Asia, resulting in more rainfall over East Asia during a ramp-down period. This result implies that the removal of CO₂ does not guarantee the return of regional rainfall to the previous climate state with the same CO₂ concentration.

Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models

For the current generation of earth system models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), the range of equilibrium climate sensitivity (ECS, a hypothetical value of global warming at equilibrium for a doubling of CO2) is 1.8°C to 5.6°C, the largest of any generation of models dating to the 1990s. Meanwhile, the range of transient climate response (TCR, the surface temperature warming around the time of CO2 doubling in a 1% per year CO2 increase simulation) for the CMIP6 models of 1.7°C (1.3°C to 3.0°C) is only slightly larger than for the CMIP3 and CMIP5 models. Here we review and synthesize the latest developments in ECS and TCR values in CMIP, compile possible reasons for the current values as supplied by the modeling groups, and highlight future directions. Cloud feedbacks and cloud-aerosol interactions are the most likely contributors to the high values and increased range of ECS in CMIP6.

Ocean Sensitivity to Periodic and Constant Volcanism

It is strongly believed that the explosive eruptions produce negative radiative forcing that causes long-term perturbations in the ocean. Moreover, it is anticipated that a sporadic strong cooling should initiate more vigorous vertical mixing of the upper ocean, and therefore cools the ocean more effectively than a uniform radiative forcing. However, the long-term simulations show that on average the ocean heat content responses to periodic and constant forcings are comparable. To better understand this controversy and to better quantify the post-eruption oceanic response, we conducted two sets of parallel simulations, the first with a uniform/constant volcanic forcing and the second one with a periodic volcanic forcing of magnitude 1×, 5×, 10× and 30× of Pinatubo size eruption using Geophysical Fluid Dynamics Laboratory’s coupled model, CM2.1. We systematically compared the effect of periodic volcanic forcing with an equivalent time-average volcanic cooling. Our results reveal that on average, volcanic-induced perturbations in Ocean Heat Content (OHC), and sea-level rise (SLR) following uniform and periodic eruptions are almost identical. It further emphasizes that the strength of ocean heat uptake at different ocean depths is mainly driven by the strength of the Atlantic Meridional Overturning Circulation (AMOC). These findings are important for ocean initialization in long-term climate studies, and geoengineering applications. It would help to unfold uncertainties related to ocean relaxation process, heat storage, and redistribution.

Time of Steady Climate Change

Under an emission scenario where atmospheric greenhouse gas concentrations are stabilized, previous work suggests that on centennial time scales the rate of global temperature increases would steady at significantly lower rates than those of the 21st century. As climate change is not globally uniform, regional differences in achieving this steady rate of warming can be expected. Here, we define a "Time of Steady Change" (TSC) as the time of reaching this steady rate of warming, and we present a method for estimating TSC with the use of General Circulation Model experiments run under greenhouse gas stabilization scenarios. We find that TSC occurs latest in low latitudes and in the Arctic, despite these areas steadying at very different absolute warming rates. These broad patterns are robust across multiple General Circulation Model ensembles and alternative definitions of TSC. These results indicate large regional differences in the trajectory of climate change in coming centuries.

Slow climate mode reconciles historical and model-based estimates of climate sensitivity

The latest Intergovernmental Panel on Climate Change Assessment Report widened the equilibrium climate sensitivity (ECS) range from 2° to 4.5°C to an updated range of 1.5° to 4.5°C in order to account for the lack of consensus between estimates based on models and historical observations. The historical ECS estimates range from 1.5° to 3°C and are derived assuming a linear radiative response to warming. A Bayesian methodology applied to 24 models, however, documents curvature in the radiative response to warming from an evolving contribution of interannual to centennial modes of radiative response. Centennial modes display stronger amplifying feedbacks and ultimately contribute 28 to 68% (90% credible interval) of equilibrium warming, yet they comprise only 1 to 7% of current warming. Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates.

High chance that current atmospheric greenhouse concentrations commit to warmings greater than 1.5 °C over land

The recent Paris UNFCCC climate meeting discussed the possibility of limiting global warming to 2 °C since pre-industrial times, or possibly even 1.5 °C, which would require major future emissions reductions. However, even if climate is stabilised at current atmospheric greenhouse gas (GHG) concentrations, those warming targets would almost certainly be surpassed in the context of mean temperature increases over land only. The reason for this is two-fold. First, current transient warming lags significantly below equilibrium or “committed” warming. Second, almost all climate models indicate warming rates over land are much higher than those for the oceans. We demonstrate this potential for high eventual temperatures over land, even for contemporary GHG levels, using a large set of climate models and for which climate sensitivities are known. Such additional land warming has implications for impacts on terrestrial ecosystems and human well-being. This suggests that even if massive and near-immediate emissions reductions occur such that atmospheric GHGs increase further by only small amounts, careful planning is needed by society to prepare for higher land temperatures in an eventual equilibrium climatic state.

Betting strategies on fluctuations in the transient response of greenhouse warming

We examine a series of betting strategies on the transient response of greenhouse warming, expressed by changes in 15-year mean global surface temperature from one 15-year period to the next. Over the last century, these bets are increasingly dominated by positive changes (warming), reflecting increasing greenhouse forcing and its rising contribution to temperature changes on this time scale. The greenhouse contribution to 15-year trends is now of a similar magnitude to typical naturally occurring 15-year trends. Negative 15-year changes (decreases) have not occurred since about 1970, and are still possible, but now rely on large, and therefore infrequent, natural variations. Model projections for even intermediate warming scenarios show very low likelihoods of obtaining negative 15-year changes over the coming century. Betting against greenhouse warming, even on these short time scales, is no longer a rational proposition.

Feedbacks, climate sensitivity and the limits of linear models

The term 'feedback' is used ubiquitously in climate research, but implies varied meanings in different contexts. From a specific process that locally affects a quantity, to a formal framework that attempts to determine a global response to a forcing, researchers use this term to separate, simplify and quantify parts of the complex Earth system. We combine new model results with a historical and educational perspective to organize existing ideas around feedbacks and linear models. Our results suggest that the state- and forcing-dependency of feedbacks are probably not appreciated enough, and not considered appropriately in many studies. A non-constant feedback parameter likely explains some of the differences in estimates of equilibrium climate sensitivity from different methods and types of data. Clarifying the value and applicability of the linear forcing feedback framework and a better quantification of feedbacks on various timescales and spatial scales remains a high priority in order to better understand past and predict future changes in the climate system.

Time-dependent climate sensitivity and the legacy of anthropogenic greenhouse gas emissions

Climate sensitivity measures the response of Earth's surface temperature to changes in forcing. The response depends on various climate processes that feed back on the initial forcing on different timescales. Understanding climate sensitivity is fundamental to reconstructing Earth's climatic history as well as predicting future climate change. On timescales shorter than centuries, only fast climate feedbacks including water vapor, lapse rate, clouds, and snow/sea ice albedo are usually considered. However, on timescales longer than millennia, the generally higher Earth system sensitivity becomes relevant, including changes in ice sheets, vegetation, ocean circulation, biogeochemical cycling, etc. Here, I introduce the time-dependent climate sensitivity, which unifies fast-feedback and Earth system sensitivity. I show that warming projections, which include a time-dependent climate sensitivity, exhibit an enhanced feedback between surface warming and ocean CO2 solubility, which in turn leads to higher atmospheric CO2 levels and further warming. Compared with earlier studies, my results predict a much longer lifetime of human-induced future warming (23,000-165,000 y), which increases the likelihood of large ice sheet melting and major sea level rise. The main point regarding the legacy of anthropogenic greenhouse gas emissions is that, even if the fast-feedback sensitivity is no more than 3 K per CO2 doubling, there will likely be additional long-term warming from slow climate feedbacks. Time-dependent climate sensitivity also helps explaining intense and prolonged warming in response to massive carbon release as documented for past events such as the Paleocene-Eocene Thermal Maximum.

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation

The covariation of carbon dioxide (CO(2)) concentration and temperature in Antarctic ice-core records suggests a close link between CO(2) and climate during the Pleistocene ice ages. The role and relative importance of CO(2) in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO(2) during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO(2) concentrations is an explanation for much of the temperature change at the end of the most recent ice age.

Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales

The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes. The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation. Antarctic climate is therefore thought to be driven by northern summer insolation. A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the 'recorder' system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.

Emergent dynamics of the climate-economy system in the Anthropocene

Global CO(2) emissions are understood to be the largest contributor to anthropogenic climate change, and have, to date, been highly correlated with economic output. However, there is likely to be a negative feedback between climate change and human wealth: economic growth is typically associated with an increase in CO(2) emissions and global warming, but the resulting climate change may lead to damages that suppress economic growth. This climate-economy feedback is assumed to be weak in standard climate change assessments. When the feedback is incorporated in a transparently simple model it reveals possible emergent behaviour in the coupled climate-economy system. Formulae are derived for the critical rates of growth of global CO(2) emissions that cause damped or long-term boom-bust oscillations in human wealth, thereby preventing a soft landing of the climate-economy system. On the basis of this model, historical rates of economic growth and decarbonization appear to put the climate-economy system in a potentially damaging oscillatory regime.

Understanding recent climate change

The Earth's atmosphere has a natural greenhouse effect, without which the global mean surface temperature would be about 33 degrees C lower and life would not be possible. Human activities have increased atmospheric concentrations of carbon dioxide, methane, and other gases in trace amounts. This has enhanced the greenhouse effect, resulting in surface warming. Were it not for the partly offsetting effects of increased aerosol concentrations, the increase in global mean surface temperature over the past 100 years would be larger than observed. Continued surface warming through the 21st century is inevitable and will likely have widespread ecological impacts. The magnitude and rate of warming for the global average will be largely dictated by the strength and direction of climate feedbacks, thermal inertia of the oceans, the rate of greenhouse gas emissions, and aerosol concentrations. Because of regional expressions of climate feedbacks, changes in atmospheric circulation, and a suite of other factors, the magnitude and rate of warming and changes in other key climate elements, such as precipitation, will not be uniform across the planet. For example, due to loss of its floating sea-ice cover, the Arctic will warm the most.

Explaining the eventual transient saturation of climate-carbon cycle feedback

BACKGROUND

Coupled climate-carbon cycle simulations generally show that climate feedbacks amplify the buildup of CO2 under respective anthropogenic emission. The effect of climate-carbon cycle feedback is characterised by the feedback gain: the relative increase in CO2 increment as compared to uncoupled simulations. According to the results of the recent Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP), the gain is expected to increase during the 21st century. This conclusion is not supported by the climate model developed at the A.M. Obukhov Institute of Atmospheric Physics at the Russian Academy of Sciences (IAP RAS CM). The latter model shows an eventual transient saturation of the feedback gain. This saturation is manifested in a change of climate-carbon cycle feedback gain which grows initially, attains a maximum, and then decreases, eventually tending to unity.

RESULTS

Numerical experiments with the IAP RAS CM as well as an analysis of the conceptual framework demonstrate that this eventual transient saturation results from the fact that transient climate sensitivity decreases with time.

CONCLUSION

One may conclude that the eventual transient saturation of the climate-carbon cycle feedback is a fundamental property of the coupled climate-carbon system that manifests itself on a relevant time scale.

Recent changes in solar outputs and the global mean surface temperature. III. Analysis of contributions to global mean air surface temperature rise

A multivariate fit to the variation in global mean surface air temperature anomaly over the past half century is presented. The fit procedure allows for the effect of response time on the waveform, amplitude and lag of each radiative forcing input, and each is allowed to have its own time constant. It is shown that the contribution of solar variability to the temperature trend since 1987 is small and downward; the best estimate is −1.3% and the 2 σ confidence level sets the uncertainty range of −0.7 to −1.9%. The result is the same if one quantifies the solar variation using galactic cosmic ray fluxes (for which the analysis can be extended back to 1953) or the most accurate total solar irradiance data composite. The rise in the global mean air surface temperatures is predominantly associated with a linear increase that represents the combined effects of changes in anthropogenic well-mixed greenhouse gases and aerosols, although, in recent decades, there is also a considerable contribution by a relative lack of major volcanic eruptions. The best estimate is that the anthropogenic factors contribute 75% of the rise since 1987, with an uncertainty range (set by the 2 σ confidence level using an AR(1) noise model) of 49–160%; thus, the uncertainty is large, but we can state that at least half of the temperature trend comes from the linear term and that this term could explain the entire rise. The results are consistent with the intergovernmental panel on climate change (IPCC) estimates of the changes in radiative forcing (given for 1961–1995) and are here combined with those estimates to find the response times, equilibrium climate sensitivities and pertinent heat capacities (i.e. the depth into the oceans to which a given radiative forcing variation penetrates) of the quasi-periodic (decadal-scale) input forcing variations. As shown by previous studies, the decadal-scale variations do not penetrate as deeply into the oceans as the longer term drifts and have shorter response times. Hence, conclusions about the response to century-scale forcing changes (and hence the associated equilibrium climate sensitivity and the temperature rise commitment) cannot be made from studies of the response to shorter period forcing changes.

Extreme events due to human-induced climate change

A recent assessment by the intergovernmental panel on climate change concluded that the Earth's climate would be 2-6 degrees C warmer than in the pre-industrial era by the end of the twenty-first century, due to human-induced increases in greenhouse gases. In the absence of other changes, this would lead to the warmest period on Earth for at least the last 1000 years, and probably the last 100,000 years. The large-scale warming is expected to be accompanied by increased frequency and/or intensity of extreme events, such as heatwaves, heavy rainfall, storms and coastal flooding. There are also several possibilities that this large change could initiate nonlinear climate responses which lead to even more extreme and rapid (on the time-scale of decades) climate change, including the collapse of the ocean 'conveyor belt' circulation, the collapse of major ice sheets or the release of large amounts of methane in high latitudes leading to further global warming. Although these catastrophic events are much more speculative than the direct warming due to increased greenhouse gases, their potential impacts are great and therefore should be included in any risk assessment of the impacts of anthropogenic climate change.

Contributions of past and present human generations to committed warming caused by carbon dioxide

We developed a highly simplified approach to estimate the contributions of the past and present human generations to the increase of atmospheric CO(2) and associated global average temperature increases. For each human generation of adopted 25-year length, we use simplified emission test cases to estimate the committed warming passed to successive children, grandchildren, and later generations. We estimate that the last and the current generation contributed approximately two thirds of the present-day CO(2)-induced warming. Because of the long time scale required for removal of CO(2) from the atmosphere as well as the time delays characteristic of physical responses of the climate system, global mean temperatures are expected to increase by several tenths of a degree for at least the next 20 years even if CO(2) emissions were immediately cut to zero; that is, there is a commitment to additional CO(2)-induced warming even in the absence of emissions. If the rate of increase of CO(2) emissions were to continue up to 2025 and then were cut to zero, a temperature increase of approximately 1.3 degrees C compared to preindustrial conditions would still occur in 2100, whereas a constant-CO(2)-emissions scenario after 2025 would more than double the 2100 warming. These calculations illustrate the manner in which each generation inherits substantial climate change caused by CO(2) emissions that occurred previously, particularly those of their parents, and shows that current CO(2) emissions will contribute significantly to the climate change of future generations.

Earth's Energy Imbalance: Confirmation and Implications

Our climate model, driven mainly by increasing human-made greenhouse gases and aerosols, among other forcings, calculates that Earth is now absorbing 0.85 +/- 0.15 watts per square meter more energy from the Sun than it is emitting to space. This imbalance is confirmed by precise measurements of increasing ocean heat content over the past 10 years. Implications include (i) the expectation of additional global warming of about 0.6 degrees C without further change of atmospheric composition; (ii) the confirmation of the climate system's lag in responding to forcings, implying the need for anticipatory actions to avoid any specified level of climate change; and (iii) the likelihood of acceleration of ice sheet disintegration and sea level rise.

The climate change commitment

Even if atmospheric composition were fixed today, global-mean temperature and sea level rise would continue due to oceanic thermal inertia. These constant-composition (CC) commitments and their uncertainties are quantified. Constant-emissions (CE) commitments are also considered. The CC warming commitment could exceed 1 degrees C. The CE warming commitment is 2 degrees to 6 degrees C by the year 2400. For sea level rise, the CC commitment is 10 centimeters per century (extreme range approximately 1 to 30 centimeters per century) and the CE commitment is 25 centimeters per century (7 to 50 centimeters per century). Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels. For sea level rise, a substantial long-term commitment may be impossible to avoid.

Constraints on radiative forcing and future climate change from observations and climate model ensembles

The assessment of uncertainties in global warming projections is often based on expert judgement, because a number of key variables in climate change are poorly quantified. In particular, the sensitivity of climate to changing greenhouse-gas concentrations in the atmosphere and the radiative forcing effects by aerosols are not well constrained, leading to large uncertainties in global warming simulations. Here we present a Monte Carlo approach to produce probabilistic climate projections, using a climate model of reduced complexity. The uncertainties in the input parameters and in the model itself are taken into account, and past observations of oceanic and atmospheric warming are used to constrain the range of realistic model responses. We obtain a probability density function for the present-day total radiative forcing, giving 1.4 to 2.4 W m-2 for the 5-95 per cent confidence range, narrowing the global-mean indirect aerosol effect to the range of 0 to -1.2 W m-2. Ensemble simulations for two illustrative emission scenarios suggest a 40 per cent probability that global-mean surface temperature increase will exceed the range predicted by the Intergovernmental Panel on Climate Change (IPCC), but only a 5 per cent probability that warming will fall below that range.

Quantifying the uncertainty in forecasts of anthropogenic climate change

Forecasts of climate change are inevitably uncertain. It is therefore essential to quantify the risk of significant departures from the predicted response to a given emission scenario. Previous analyses of this risk have been based either on expert opinion, perturbation analysis of simplified climate models or the comparison of predictions from general circulation models. Recent observed changes that appear to be attributable to human influence provide a powerful constraint on the uncertainties in multi-decadal forecasts. Here we assess the range of warming rates over the coming 50 years that are consistent with the observed near-surface temperature record as well as with the overall patterns of response predicted by several general circulation models. We expect global mean temperatures in the decade 2036-46 to be 1-2.5 K warmer than in pre-industrial times under a 'business as usual' emission scenario. This range is relatively robust to errors in the models' climate sensitivity, rate of oceanic heat uptake or global response to sulphate aerosols as long as these errors are persistent over time. Substantial changes in the current balance of greenhouse warming and sulphate aerosol cooling would, however, increase the uncertainty. Unlike 50-year warming rates, the final equilibrium warming after the atmospheric composition stabilizes remains very uncertain, despite the evidence provided by the emerging signal.

Global warming in the twenty-first century: an alternative scenario

A common view is that the current global warming rate will continue or accelerate. But we argue that rapid warming in recent decades has been driven mainly by non-CO(2) greenhouse gases (GHGs), such as chlorofluorocarbons, CH(4), and N(2)O, not by the products of fossil fuel burning, CO(2) and aerosols, the positive and negative climate forcings of which are partially offsetting. The growth rate of non-CO(2) GHGs has declined in the past decade. If sources of CH(4) and O(3) precursors were reduced in the future, the change in climate forcing by non-CO(2) GHGs in the next 50 years could be near zero. Combined with a reduction of black carbon emissions and plausible success in slowing CO(2) emissions, this reduction of non-CO(2) GHGs could lead to a decline in the rate of global warming, reducing the danger of dramatic climate change. Such a focus on air pollution has practical benefits that unite the interests of developed and developing countries. However, assessment of ongoing and future climate change requires composition-specific long-term global monitoring of aerosol properties.

Can increasing carbon dioxide cause climate change?

The realistic physical functioning of the greenhouse effect is reviewed, and the role of dynamic transport and water vapor is identified. Model errors and uncertainties are quantitatively compared with the forcing due to doubling CO2, and they are shown to be too large for reliable model evaluations of climate sensitivities. The possibility of directly measuring climate sensitivity is reviewed. A direct approach using satellite data to relate changes in globally averaged radiative flux changes at the top of the atmosphere to naturally occurring changes in global mean temperature is described. Indirect approaches to evaluating climate sensitivity involving the response to volcanic eruptions and Eocene climate change are also described. Finally, it is explained how, in principle, a climate that is insensitive to gross radiative forcing as produced by doubling CO2 might still be able to undergo major changes of the sort associated with ice ages and equable climates.

On the scientific basis for global warming scenarios

The scientific basis for current projections of significant warming due to enhanced minor greenhouse gases in the atmosphere is reviewed. Care is taken to distinguish the issue of changes in radiative forcing at the earth's surface from the issue of the climatic response to this forcing. With respect to the former, it is noted that the predicted forcing is, in fact, small (2 W m(-2) at the surface for a doubling of CO(2), or less than 1% of the absorbed solar flux). With respect to the latter, it is noted that predictions of significant warming are dependent on the presence of large positive feedbacks serving to amplify the response. The largest of these feedbacks in current models involves water vapor at upper levels in the troposphere. This feedback appears to be largely a model artifact, and evidence is presented that models may even have the wrong sign for this feedback. The possibility is examined that the response of climate to major volcanic eruptions may provide a test of the climate system's amplification. The basis for this possibility is the fact that the response delay of the ocean-atmosphere system is proportional to the system gain.

Climate of the earth: an overview

Climate has varied over a large range in the recent history of the Earth, with extremes represented by equable environments of the Cretaceous and Eocene and the comparatively frigid conditions of the ice ages that punctuated the past few million years. It is suggested that major shifts in climate are controlled largely by variations in CO(2) with related fluctuations in modes of ocean circulation. Changes in climate can proceed rapidly, on time scales as short as centuries or even decades, as indicated by data for the Younger Dryas (a period of globally cold conditions interrupting recovery of the Earth from the last ice age) and the Little Ice Age (a cold snap extending from about 1250 to about 1850 ad). Rapid fluctuations in climate appear to be linked to changes in production of deep water in the North Atlantic, possibly also to variations in circulation of intermediate waters in the Pacific. Mechanisms are discussed whereby changes in ocean circulation can result in shifts of climate on a global scale. A 10 000 year record of climate from Norway is used to provide context for a discussion of possible changes in climate today arising as a result of the build-up of industrially related greenhouse gases. Brief, somewhat pessimistic, comments are offered concerning the prospects for meaningful near-term predictions of the response of climate to increased concentrations of greenhouse gases. Studies of past climates, by drawing attention to important processes and feedbacks, can play a valuable role in the development of credible models for the future.