SIO40lecture24

SIO40lecture24 - SIO 40 Life and Climate on Earth Dec. 1,...

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SIO 40 – Life and Climate on Earth Dec. 1, 2010 Lecture 24 – Mitigating global warming, Part II Reducing emissions
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Actual emission trends vs. IPCC AR4 scenarios The global rate of increase of fossil fuel CO2 emissions has accelerated three-fold over the last 18 years, increasing from 1.0% per year in the 1990s to 3.4% per year between 2000-2008 (see above). Global CO 2 emissions from fossil fuel burning are thus tracking near the highest emission scenarios considered so far by the IPCC.
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Temperature projections based on emissions scenarios The latest estimates of global mean air temperature projected out to 2100 are shown above. The wide range in the projection envelope is primarily due to uncertainty in future emissions. At the high end of emissions, which is how the trend is going now (see previous slide), global mean warming is estimated to reach 4-7°C by 2100, which will be quite an extreme climate change for human societies and global ecosystems relative to the past 2000 years (see above). A 2-3 ° C temperature change would be easier to cope with, but would require significant and lasting emission cuts, starting in the very near future.
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It is important to note that even if CO 2 emissions were to reach a maximum and then suddenly stop altogether, the CO 2 already emitted will remain in the atmosphere (top left) and global surface temperatures will remain elevated (bottom left) for at least the next 1000 years - essentially forever in human terms!
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Most of the various natural sinks for atmospheric CO 2 operate relatively slowly The long lifetime of CO 2 in the atmosphere (hundreds to thousands of years) is due to the fact that the natural processes that take CO 2 out of the atmosphere (eg. ocean uptake, weathering reactions, etc) operate over hundreds to thousands of years. Thus, while we might conceivably be able to stop global warming if we could somehow stop emitting CO 2 , it would be very hard to reverse global warming, unless we devised a way to quickly extract vast quantities of CO 2 from the atmosphere. Viewed in this light, it might make sense to take steps to limit CO 2 emissions.
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Emission trends needed in order to limit global warming to about 2 ° C The most widely supported policy goal is to limit global warming to at most 2 °C above the preindustrial temperature level. A number of recent scientific studies have investigated in detail what global emissions trajectories would be compatible with limiting global warming to 2 °C. Above is shown examples of global emission pathways where cumulative CO2 emissions equal 750 Gt during the time period 2010-2050. At this level, there is a 67% probability of limiting global warming to a maximum of 2°C. The graph shows that the later the peak in emissions is reached, the steeper their subsequent reduction has to be. The figure shows variants of a global emissions scenario with different peak years: 2011 (green), 2015 (blue) and 2020 (red). In order to achieve compliance with these curves, maximum annual reduction rates of 3.7 % (green), 5.3 % (blue) or 9.0 % (red) would be required (relative to 2008).
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This note was uploaded on 05/02/2011 for the course SIO 40 taught by Professor Barbeau,k during the Fall '08 term at UCSD.

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SIO40lecture24 - SIO 40 Life and Climate on Earth Dec. 1,...

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