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Berkeley ENVECON 131 - The science and economics of Climate Change

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The science and economics of Climate ChangeNovember 30, 2006Topics• The science of greenhouse gasses.• Modeling climate change• The economics of climate changeThe gasses• Major greenhouse gasses Methane, Nitrous Oxide, Carbon Dioxide. • The global warming potential, per molecule, of Methane 2 is 25 times greater than a molecule of CO2, and global warming potential of a molecule Nitrous Oxide is 200 greater than a molecule of CO2.• CO2 contributes more than half the increase in "relative forcing" over next 100 years, and it lasts much longer than the other gasses. It may be easier to regulate.• Photosynthesis traps carbon, burning fuel releases it.• Net primary production (the annual production of new plant tissue) captures 105 gigatons per year (54% on land and rest at sea)The oceans• The oceans absorb CO2 by mixing surface water into the deep ocean.• Thermohaline circuit (THC) is a major current system in the Atlantic Ocean.• Gulf stream takes warm salty water from southern hemisphere northward, along the East coast of US and then northeast to Greenland, Iceland and Norway.• The warm surface water is cooled by the cold air, increasing the density of the water and causing it to sink, creating a kind of vacuum which draws warm salty water from Gulf of Mexico, warming the air over the European continent.Weakening of Thermohaline Circuit?• Greater snowmelts due to global warming, increasing the flow of cold freshwater (which is lighter than salt water) could reduce the conveyor effect (because there is less "sinking" of water), leading to a smaller vacuum effect.• Cooler sea temperature could cause ice to form, creating a kind of lid over ocean, reducing the amount of cooling of the water, reducing the vacuum effect. The reflectivity (albedo) of the ice leads to more cooling in Europe• Although a possible weakening or shutdown of THC is widely discussed in science community, there is disagreement about the likelihood of this event.The time-line of GHG (Greenhouse Gas) concentrations• 70 million years ago, atmospheric CO2 concentrations were several times current levels.• Atmospheric CO2 has ranged from 100 to 400 ppm over the past 20 million years. It has ranged from 180 - 290 ppm over the past 420,000 years, and between 270 -290 ppm over last 2000 years, prior to industrial revolution.• Current level about 380 ppm, an increase of about 30% from pre-industrial levels. About 75% of this increase due to fossil fuel burning, the remainder due to deforestation and industrial processes.• 150% increase in methane concentration since pre-industrial, about half of which is anthropogenic. Nitrous oxide up about 17%.Current emissions and projections• IPCC projections show CO2 increases to 550 ppm in 2050 and greater than 700 ppm by 2100 under BAU.• Current carbon emissions from burning fuel about 6.3 gigaton, projected to rise to 15 gigaton by 2050.• We are currently removing carbon from earth's crust at a rate 100 times greater than the natural storage in marine sediments. • There is mixed evidence whether forests are a net source or sink of carbon. Deforestation contributes about 1.6 gigaton per year, but there is substantial reforestation in northern latitudes.Carbon fertilization and deforestation• More carbon in the atmosphere increases the rate of photosynthesis. Some damage estimate models assume that carbon fertilization substantially increases plant productivity (a benefit of global warming).• Carbon fertilization effect has been documented in short run experiments, but in the long run, as plants are able to adapt, it does not result in greater plant mass. Experiments found an initial 25% increase in plant growth of young pine trees, but after 4 years there was no statistically significant effect on growth.• It would require reforestation of all land on earth (including agricultural and urban land) to store 6 gigatons of carbon a year.Attempts to model climate change• "Climate change" means a change in the distribution of weather. • Small changes in the earth's orbit over 10s of millions of years have been linked to changes in the size of continental ice sheets on Greenland and Antarctica.• Climate change models partition sphere into between 450 to 45,000 boxes (a grid of between 100 and 1000 kilometers in length). Portion atmosphere into 10 to 100 layers, resulting in 4,500 to 4.5 million cells.• Similar, but finer partition of oceans. Even this level of detail is not fine enough to capture many important phenomena.Modeling feedback effects• Increase in water vapor in atmosphere leads to further warming, which might magnify an exogenous increase in temperature by as much as a factor of 2.• Melting of ice sheets diminishes reflectivity, increasing absorption of heat; this effect could amplify an exogenous temperature increase by a few tens of percent. (Spring snowmelt in Artic tundra has increased about 2.5 days per decade since the 1960s.)Feedbacks, continued• The sign of the feedback effect of clouds is uncertain, but often thought to be positive. There is not a unique way of incorporating these feedback effects into models; they occur at the subgrid level, so different models can give different results.• Higher temperatures would cause more rapid decomposition in soils, leading to faster carbon production. The cold and waterlogged conditions in in boreal and tundra soils causes the accumulation of organic matter. A slight warming could lead to the faster decomposition of this material, and greater release of carbon.Measures of recent changes• Data from thermometers show that the earth's global average surface temperature has increased by about 0.6 degrees Celsius over the past 100 years, and sea-level has increased by 0.1 to 0.2 meters. • Frequency of Atlantic hurricanes has not changed, but the number of category 4 and 5 level storms has doubled in the past 35 years.Climate model assumptions• IPCC report develops over 40 different scenarios of emissions. (About 75% of emissions remain in atmosphere.) The different scenarios show concentration levels in 2100 from about 500 ppm to 1000 ppm. • A rough estimate is that over the next century concentrations could double from current levels (reaching 760 ppm).• Climate modelers use these emissions scenarios as inputs into their models to predict changes in temperature and sea level.Climate model results• Using emissions scenarios and two


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Berkeley ENVECON 131 - The science and economics of Climate Change

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