Dangerous human-made interference with climate: a GISS modelE study

Abstract

Abstract. We investigate the issue of "dangerous human-made interference with climate" using simulations with GISS modelE driven by measured or estimated forcings for 1880–2003 and extended to 2100 for IPCC greenhouse gas scenarios as well as the "alternative" scenario of Hansen and Sato (2004). Identification of "dangerous" effects is partly subjective, but we find evidence that added global warming of more than 1°C above the level in 2000 has effects that may be highly disruptive. The alternative scenario, with peak added forcing ~1.5 W/m2 in 2100, keeps further global warming under 1°C if climate sensitivity is ~3°C or less for doubled CO2. The alternative scenario keeps mean regional seasonal warming within 2σ (standard deviations) of 20th century variability, but other scenarios yield regional changes of 5–10σ, i.e., mean conditions outside the range of local experience. We discuss three specific sub-global topics: Arctic climate change, tropical storm intensification, and ice sheet stability. We suggest that Arctic climate change has been driven as much by pollutants (O3, its precursor CH4, and soot) as by CO2, offering hope that dual efforts to reduce pollutants and slow CO2 growth could minimize Arctic change. Simulated recent ocean warming in the region of Atlantic hurricane formation is comparable to observations, suggesting that greenhouse gases (GHGs) may have contributed to a trend toward greater hurricane intensities. Increasing GHGs cause significant warming in our model in submarine regions of ice shelves and shallow methane hydrates, raising concern about the potential for accelerating sea level rise and future positive feedback from methane release. Growth of non-CO2 forcings has slowed in recent years, but CO2 emissions are now surging well above the alternative scenario. Prompt actions to slow CO2 emissions and decrease non-CO2 forcings are needed to achieve the low forcing of the alternative scenario.

Climate change scenarios for the California region

“…By 2070-2099, in all of the projections, temperature increases were sufficient to nearly eliminate seasonal mean temperatures in the lower third of the distribution and sharply reduce those in the middle third. Such climate changes would be, in the words of Hansen et al 2007, "climate changes outside of the range of local experience." A noteworthy feature in the temperature projections is that the warming through the twenty-first Century does not level off, especially in projections using the medium and high greenhouse gas emission scenarios, implying that California's climate would continue to warm in (at least) the subsequent decades of the twenty-second century.…”

Assessing the strength of regional changes in near-surface climate associated with a global warming of 2°C

“…The Alternative scenario ends at a CO 2 concentration of 475 ppm in 2100. In Hansen et al (2007) the GISS modelE coupled climate model was forced with this scenario, and it was found that the Alternative scenario kept the future global warming below 1 • C with respect to the end of the twentieth century for a climate sensitivity of about 3 • C or less for doubled CO 2 . May (2008) used a somewhat different approach.…”

Subjective elements in climate policy advice

“…However, even use of multiple pdf's tends to keep the focus on planning for the most likely outcomes at the expense of the extremes (unless the pdf's are dissimilar). This can be dangerous when the stakes associated with the extremes (the upper end of climate sensitivity estimates in this case) are so high (Schneider 1997;Kasting 1998;Hansen 2005;Hansen et al 2007). We need to supplement pdf's as a tool for communicating climate sensitivity with another method that is capable of highlighting extremes.…”