Hansen et al 2006 Global Temperature Changes

There is strong evidence that the wep sst during the

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Unformatted text preview: olocene peak values. However, as shown in Figs. 1B and 3A, those are exceptional regions. Most of the world and the global mean have warmed as much as the WEP and Indian Oceans. We infer that global temperature today is probably at or near its highest level in the Holocene. Fig. 5 shows that recent warming of the WEP has brought its temperature within 1°C of its maximum in the past million years. There is strong evidence that the WEP SST during the penultimate interglacial period, marine isotope stage (MIS) 5e, exceeded the WEP SST in the Holocene by 1–2°C (30, 31). This evidence is consistent with data in Figs. 4 and 5 and with our conclusion that the Earth is now within 1°C of its maximum temperature in the past million years, because recent warming has lifted the current temperature out of the prior Holocene range. Criteria for Dangerous Warming. The United Nations Framework Convention on Climate Change (www.unfccc.int) has the objective ‘‘to achieve stabilization of GHG concentrations’’ at a level preventing ‘‘dangerous anthropogenic interference’’ (DAI) with climate, but climate change constituting DAI is undefined. We suggest that global temperature is a useful metric to assess proximity to DAI, because, with knowledge of the Earth’s history, global temperature can be related to principal dangers that the Earth faces. We propose that two foci in defining DAI should be sea level and extinction of species, because of their potential tragic consequences and practical irreversibility on human time scales. In considering these topics, we find it useful to contrast two distinct scenarios abbreviated as ‘‘business-as-usual’’ (BAU) and the ‘‘alternative scenario’’ (AS). BAU has growth of climate forcings as in intermediate or strong Intergovernmental Panel on Climate Change scenarios, such as A1B or A2 (10). CO2 emissions in BAU scenarios continue to grow at 2% per year in the first half of this century, and non-CO2 positive forcings such as CH4, N2O, O3, and black carbon (BC) aerosols also continue to grow (10). BAU, with nominal climate sensitivity (3 1°C for doubled CO2), yields global warming (above year 2000 level) of at least 2–3°C by 2100 (10, 17). AS has declining CO2 emissions and an absolute decrease of non-CO2 climate forcings, chosen such that, with nominal climate sensitivity, global warming (above year 2000) remains 1°C. For example, one specific combination of forcings has CO2 peaking at 475 ppm in 2100 and a decrease of CH4, O3, and BC sufficient to balance positive forcing from increase of N2O and decrease of sulfate aerosols. If CH4, O3, and BC do not decrease, the CO2 cap in AS must be lower. Sea level implications of BAU and AS scenarios can be considered in two parts: equilibrium (long-term) sea level change and ice sheet response time. Global warming 1°C in AS keeps temperatures near the peak of the warmest interglacial periods of the past million years. Sea level may have been a few meters higher than...
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This document was uploaded on 03/15/2014 for the course MEA 570 at N.C. State.

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