Climatic Change: What Can We Hope To Know? Starley L. Thompson Climate and Carbon Cycle Modeling Group Atmospheric Science Division Energy and Environment Directorate Lawrence Livermore National Laboratory

Contents • Global Warming: The observational evidence • Causes of Climate Change: Natural and anthropogenic causes • The Future: Predictions and The Big Unknowns

The Earth’s Climate is Changing • Global average increase of 0.6 ºC over the 20th century • 1990s were the warmest decade in the instrumental record Observed Change in Global Mean Temperature Since 1860

The Earth’s Climate is Changing • The increase in temperature during the 20th century is likely to have been the largest of any century during the past 1000 years. Estimated Change in Northern Hemisphere Temperature Since 1000 AD

A Collective Picture of a Warming World • Global sea level rise of 0.1 to 0.2 meters in the 20th century • Length of freeze-free season has lengthened in mid and high latitudes • Reduction in the frequency of extreme low temperatures since 1950 • 10% decrease in land snow cover since 1960s • Retreat of mountain glaciers in the 20th century • 2-4% increase in frequency of heavy precipitation events in mid and high latitudes of the northern hemisphere • 10-15% decrease in summer sea ice extent since 1950s • A few areas have not warmed, mainly some parts of the southern hemisphere oceans and Antarctica

Carbon Dioxide • Atmospheric CO2 has increased by 31% since 1750. • The present amount of CO2 has likely not been exceeded during the past 20 million years • The current rate of increase of CO2 is unprecedented in the past 20,000 years. Hawaii Ice Cores A Cause of Global Warming? Atmospheric CO2 for the past millenium.

Human Influence on the Atmosphere During the Industrial Era Carbon Dioxide Nitrous Oxide Methane Concentrations of three well-mixed greenhouse gases • Atmospheric methane has increased by 151% since 1750. About half of current emissions are anthropogenic. • Atmospheric nitrous oxide has increased by 17% since 1750. About a third of current emissions are anthropogenic.

Human Influence on the Atmosphere During the Industrial Era Sulphate Aerosols Simulated global sulphate aerosol distribution for October Measured sulphate in Greenland ice and SO2 emissions since 1900

CO2 currently accounts for 60% of the radiative forcing from well-mixed greenhouse gases Warming Cooling

Simulating Global Warming A standard procedure is to increase the carbon dioxide (CO2) in a climate model simulation and then compare the results to a control run without a CO2 increase. NOTE: Atmospheric CO2 is expected to double well before the end of the 21st century. Change in annual mean temperature (ºC) resulting from a doubling of CO2 in the atmosphere. Source: Govindasamy and Caldeira (2000)

Climate as a Computational Problem Building a Climate Model? You will need • Equations of fluid motion and thermodynamics Applied on • A four dimensional global grid (3 space + 1 time) Including • “Physics” (e.g., clouds, sunlight, etc.) Then, combine together and run on a rather large computer. The atmosphere and oceans are fluids on a rotating sphere. Image Copyright  1987 by Fulcrum, Inc.

The Limits to Climate Computation Climate is Derived From Weather • Climate modeling is really weather modeling • We start from an initial state and compute forward in time by small steps. Each halving of grid spacing requires eight times more computer power. Latitude Height Longitude Unresolved Processes • Clouds and precipitation occur on spatial scales much finer than the explicit computational grid. An example of how the Earth is spatially gridded in current operational climate models

How Do We Know The Models Are Right? We cannot know with certainty until we perform the experiment, but we can do proxy validation to increase our confidence. Model Intercomparison Historical climate of the 19 & 20th centuries Detailed records post WW II era Paleoclimates geologic past Confidence In Model Results Testing Individual Model Components Q: How can we predict climate if we can’t predict the weather? A: The chaotic details of weather are not predictable beyond several days, but the statistics of climate are, in principle, predictable.

Testing by Simulating the 20th Century Natural Forcing Only • Natural (solar + volcanic) forcing alone does not account for warming in the past 50 years. • Adding human influences (greenhouse gases and sulphate aerosols) brings the models and observations into pretty good agreement. Anthropogenic Forcing Only Natural and Anthropogenic Forcing Testing by Simulating the 20th Century

Projecting Into The Future: CO2 Increase Scenarios • Emissions of CO2 due to fossil fuel burning will be the dominant influence on atmospheric CO2 in the 21st century • Stabilization of CO2 at twice the pre-industrial level will require emissions to drop to below 1990 levels in less than 50 years. • Emissions will need to continue to decrease steadily thereafter to a very small fraction of current emissions.

Projecting Into The Future: Temperature Change • Global Increase Ranging From 1.4 to 5.8 ºC from 1990 to 2100. • The rate of warming is very likely to be without precedent in at least the last 10,000 years • The range of uncertainty due to climate models is comparable to the range of uncertainty due to CO2 scenarios. • It is very likely that nearly all land areas will warm more rapidly than the global average.

Extreme Weather and Climate Events

A Grand Challenge: Climate and Carbon Cycle Interaction • Human activities perturb the natural carbon cycle. • We put about 8 gigatons of carbon (as CO2) into the atmosphere per year: Manmade Source Gt C/yr -------------------------- --------- Fossil Fuel Burning: 6 Cement Production: 0.1 Land Use Change: 1.7 • About half the emitted CO2 accumulates in the atmosphere. The remainder is taken up by the land and oceans. Fossil fuel use: 6 Atmosphere: 730 Yearly Increase: 4 Deforestation: 2 Net 120 122 2 Net Vegetation: 500 102 104 Soil: 1500 2 Ocean: 38,000 Recoverable Fossil Fuel: 10,000 The Global Carbon Cycle. Numbers indicate annual flows and reservoirs in gigatons of carbon.

The Global Carbon Cycle Is Sensitive to Climate Change Climate change will affect the uptake of anthropogenic carbon dioxide by the oceans and terrestrial biosphere. -8.0 3.5 Major Carbon Sink Regions in Oceans Simulated interannual variability of carbon uptake by terrestrial vegetation arising from interannual variability of sea surface temperatures Present carbon uptake by the oceans (Moles CO2 /square meter/year)

Moving from Specified to Predicted CO2 • Currently, projections of climate change do this: Specified Atmospheric CO2 Concentration Climate Model Future Climate • More credible projections will need to do this: Combined Climate and Carbon Cycle Model CO2 Concentration Specified CO2 Emissions Future Climate

Beyond Coupling: Making the Models Useful • The current coarse resolution of climate models hampers their utility for climate change impacts studies. • The models need to provide information relevant to planners and policymakers. Achieving a Congressional Resolution: Current Global Climate Model Grid Area Average Area of a Congressional District 300 km 150 km

A Big Unknown: Clouds and Water Vapor Changes in Climate Models Coarse Spatial Grids Used in Typical Climate Models Result in the Use of Approximations for Clouds and Rain The global mean temperature response of climate models to a doubling of CO2 varies from about 1.5 to 4.5 ºC. This range of uncertainty is mostly due to climate feedbacks from water vapor and clouds Topography at 300 km grid resolution

Response of Ocean Circulation to Global Warming The overturning circulation is driven by the sinking of dense water at high latitudes Dots mark sites of deep water formation Density is determined by temperature and salinity, thus the term thermohaline circulation (THC) The overturning THC component in the Atlantic Ocean is important for northern hemisphere heat balance Source: Rahmstorf, Nature (1999)

Two Stable Climate States? Climate models can produce two stable states for the THC strength No THC (thermohaline collapse) is one possible state. Similar behavior is seen in the geologic record.

Global Warming and THC Collapse Anything that acts to decrease water density at Atlantic high latitudes can promote THC collapse. Global warming can do this in two ways: Increase water temperature at high latitudes Increase freshwater input to high latitudes

Uncertainty in the Zone of THC Collapse 1.0 3.0oC Sensitivity, Weak Start 3.0oC Sensitivity, Normal Start 0.5 4.5oC Sensitivity, Normal Start CO2 Increase Rate (% per year) Zone of Recovery Possible Zones of Collapse 400 600 800 1000 1200 1400 Stabilized CO2 Concentration (ppmv) Collapse of the North Atlantic thermohaline circulation (THC) is more likely at higher stabilized CO2 concentrations and higher rates of CO2 increase

Summary • Global Warming: Happening now. 0.6 ºC so far. 1.4 to 5.8 ºC by 2100. • Climate Predictions: Harder to predict regional details and extreme events • Big Unknowns in Climate Processes: Clouds and water vapor Coupled carbon cycle and climate feedbacks Ocean thermohaline circulation response