Global Climate Models GCMs Changes in GCMs since

Global Climate Models (GCMs) Changes in GCMs since 1970 s How do We Know GCMs are Reliable? Limitations of GCMs Model Projections For Next Class: Continue Reading Ch. 10

What are Models? Model - approximate representation or simulation of a real system, incorporating only the essential features, or variables, of a system while omitting details considered non-essential or non-predictable Models are only as good as the input data Can be categorized as conceptual, graphical, physical or numerical Conceptual models explore linkages among physical and biological sub-systems Graphical models compile and display data in a format that readily conveys meaning Physical models are a small-scale portrayal of a system Numerical models have essentially replaced physical models in investigating the climate system

What are Models? Global Climate Models Global climate models (GCMs) - simulations of Earth’s climate system using mathematics One type consists of equations that describe the physical interactions among the various components of the climate system; simulates the circulation of the atmosphere, plus numerous processes that control energy transference and exchange. Referred to as general circulation models Fragment a map of Earth into gridded spatial coordinates resulting in a three-dimensional representation of the climate system ECV variables are computed at each grid point and predicted into the future Time step - an interval between one set of solutions and the next, and is a function of the grid spacing utilized in the GCMs

What are Models? Global Climate Models

Huge improvements in model resolution since FAR!

. . . and in modeling the physical processes!

What are Models? Model Limits • Coupling – when various subsystems of climate are modeled in parallel to another

What are Models? Global Climate Models • Physical evolution of some systems (e. g. , the orbit of the Moon) can be determined precisely by solving a specific mathematical equation • Other systems, such as the fluid atmosphere or ocean, are too complex to be described by a single mathematical formula. In these cases, statistics approximates the behavior of the system on a computer – Parameterization - technique of using estimates for small processes on a larger scale in modeling

It is very difficult to accurately parameterize clouds and cloud microphysical processes in weather or climate models!

What are Models? Weather versus Climate Models • Climate simulations are not the same as weather simulations • Climate prediction is a boundary value problem whereas weather forecasting is an initial value problem – Numerical weather prediction (NWP) models start off with conditions that represent the present and then applies the laws of physics to work forward to generate a prediction • Future state of atmosphere on spacing of a few kilometers – GCMs use fundamental boundary conditions (forcing of the Sun, GHG concentration, etc. ) and then predicts how the climate adjusts to these differing conditions • Specifies broad regions of positive/negative anomalies and mean locations of atmospheric circulation features (jet streams) • Driven by feedbacks because of long-term time scale • Don’t reference specific temperature values; instead reference summary statistics

What are Models? Model Limits • Chaos theory - changes in these systems are highly sensitive to the conditions of their initial state so small differences in their initial conditions can lead to vastly different outcomes over long periods of time – Edward Lorenz – ‘Butterfly effect’ • Models will never be perfect due to these limitations; they can be improved upon • IPCC report scientists note marked improvements in model skill through time • Lingering sources of uncertainty: – – Volcanic eruptions Ocean ECVs Small-scale weather systems Clouds

What are Models? Physical Basis for modeling Earth’s Climate • Modeling the climate is difficult – Different spatial scales – Feedbacks – No single equation that explains behavior of all interacting parts • Numerical approximations are the best possible manner to simulate the entire climate system and test its limits and sensitivities to external forcing • Used to predict climatic impacts of rising CO 2 (or other GHG) – Using current boundary conditions, GCMs simulate the present climate and then, holding constant all other variables in the model, the concentration of atmospheric CO 2 is elevated and the model is run to a new state of equilibrium – Transient run - CO 2 is slowly added to the model and the effects are evaluated from moment to moment – Equilibrium run - CO 2 is added all at once and the model is run until it achieves a new equilibrium • New climate state is compared with present

Search for Cycles and Analogs Enhanced Greenhouse Effect and Global Warming • What do GCMs indicate about climate in the near term? • If all other boundary conditions remain fixed, rising concentrations of CO 2 and other greenhouse gases would cause global warming to persist throughout this century and well beyond – Magnitude of warming depends on future emissions and feedbacks – Change is geographically nonuniform • Polar temperature amplification

Search for Cycles and Analogs Enhanced Greenhouse Effect and Global Warming • Even if greenhouse gas emissions were to stabilize at present levels, global warming would likely continue well beyond the 21 st century • Total U. S. emissions have increased by 4. 7% from 1990 to 2012 decreased by 3. 4% from 2011 to 2012 (EPA, 2014) – Shift in power plants to use less coal and more natural gas

Search for Cycles and Analogs Enhanced Greenhouse Effect and Global Warming • How the atmospheric concentration of a gas responds to a decrease in emissions depends on the competition between the rate of emission of the gas into the atmosphere and the rates of physical, chemical and biological processes that remove the gas – Interactions determine the atmospheric lifetime of the gas in the atmosphere • Time it takes for a perturbation of the gas to be reduced to 37% of its original amount – N 2 O has a lifetime of 110 years so an emission reduction of more than 50% would stabilize its atmospheric concentration near present-day values. – CH 4 has a lifetime of only 12 years, allowing a reduction by less than 30% to stabilize its atmospheric concentration within a few decades • Methane degrades to carbon dioxide, causing subsequent warming – More complicated for CO 2

Search for Cycles and Analogs Enhanced Greenhouse Effect and Global Warming • More than 50% of CO 2 emitted into the atmosphere cycles out within a century but about 20% remains for millennia – If stabilized at current levels, CO 2 concentration would continue to increase beyond this century – Only complete elimination of anthropogenic CO 2 emissions would stabilize at current levels

Search for Cycles and Analogs Climate Sensitivity • • Global radiation budget is not in equilibrium Earth now emits about 0. 85 W/m 2 less radiant energy than it receives from the Sun (imbalance of approximately 0. 06% of the total incoming solar radiation at the top of the atmosphere) (2005) – Increase in the heat content of the upper ocean over the previous decade – Response of this imbalance may be delayed as Earth’s ECVs (e. g. , surface temperature) take time to adjust (shifts to a new equilibrium) • Climate sensitivity - the equalizing temperature change in response to changes in external forces of the boundary conditions of climate – When there are changes in solar radiation or CO 2 concentrations for example, the climate system responds with a corresponding change in temperature to match the initial change • Scientists are asking how much and how quickly global temperatures need to rise to match this change in boundary condition – Climate sensitivity was assessed to be in the range of 2. 1 °C to 4. 7 °C, given a widely accepted projected increase in atmospheric CO 2 concentration (IPCC)

Search for Cycles and Analogs Climate Sensitivity • In many experiments involving GCMs, CO 2 concentration is elevated while all other boundary conditions are kept constant -> may not be realistic – Many interacting forcing agents and mechanisms have shaped the recent climate – Rapid rise in atmospheric CO 2 concentration was not accompanied by a consistent rise in global mean temperature over the same period – Fluctuations may have been in response to large-scale climate oscillations rather than a cause

Search for Cycles and Analogs Climate Sensitivity • Current rise in CO 2 due to anthropogenic activities is proceeding at a much more rapid pace than anytime during the Pleistocene Epoch – Reason to believe that present warming is a response to more CO 2 rather than the cause

Assessment of Climate Model Output • Why can climate models be trusted? – Replicate well the general features of the global-scale annual mean surface temperature increase over the historical period of record – Also do well duplicating the more rapid warming in the second half of the 20 th century – And the cooling immediately following large volcanic eruptions • Model strengths/weaknesses – Simulate surface temperatures better than precipitation events – Extreme climate events is another area of improvement • Occurrence of weather or climate events above threshold value for the upper range, or below the lower range, of observed values (IPCC) • Model output of changes in frequency of extreme hot and cold days and nights over the second half of the 20 th century are consistent with observations