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

Abstract

Abstract. We investigate the issue of "dangerous humanmade 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/m 2 in 2100, keeps further global warming under 1 • C if climate sensitivity is ∼3 • C or less for doubled CO 2 . 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 conclude that a CO 2 level exceeding about 450 ppm is "dangerous", but reduction of non-CO 2 forcings can provide modest relief on the CO 2 constraint. We discuss three specific sub-global topics: Arctic climate change, Correspondence to: J. Hansen (jhansen@giss.nasa.gov) tropical storm intensification, and ice sheet stability. We suggest that Arctic climate change has been driven as much by pollutants (O 3 , its precursor CH 4 , and soot) as by CO 2 , offering hope that dual efforts to reduce pollutants and slow CO 2 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-CO 2 forcings has slowed in recent years, but CO 2 emissions are now surging well above the alternative scenario. Prompt actions to slow CO 2 emissions and decrease non-CO 2 forcings are required to achieve the low forcing of the alternative scenario.

Driving forces of global wildfires over the past millennium and the forthcoming century

“…Overall, the model captures historical trends influenced by a variety of natural and anthropogenic factors remarkably well, inspiring some confidence in the model's projection of future fires. GISS GCM climate simulations (19), like other models, predict a significant warming over the forthcoming century (Fig. 2B).…”

“…8 for detailed method description and evaluation) are based on temperature, precipitation, relative humidity, and lightning activity generated in AR4 GISS GCM climate simulations and HYDE (past) and Integrated Model to Assess the Global Environment (IMAGE) (future) land-cover and population density datasets. Climate during the 844-1880 CE simulations (9) was driven by variations in solar irradiance [responsible for the vast majority of multidecadal time-scale forced variability (30)]; 1880-2003 simulations (10) were driven by multiple forcings, including greenhouse gases, tropospheric aerosols, ozone, solar irradiance, and volcanic aerosols; 2004-2100 simulations (19) were driven by changes in the wellmixed greenhouse gases [the dominant climate forcing over the past few decades (19)]. Anthropogenic ignition sources are calculated as an increasing function of population density while assuming that people living in sparsely populated regions interact more with natural ecosystems and therefore produce potentially more ignitions (31).…”

Climate change and trace gases

“…However, the climate system's long response time and slow mixing of heat into the ocean are a mixed benefit to society. Warming of the deep ocean may have at least two long-range detrimental effects (Hansen et al 2006b): erosion of ice shelves around Antarctica and Greenland (Rignot & Jacobs 2002), and destabilization of methane hydrates on continental shelves (Harvey & Huang 1995;Archer 2007). The ocean's slow response delays such effects, but there is the danger of setting in motion a warming of the deep ocean that will lock in disastrous impacts which will unfold for future generations.…”

Comparing high resolution WRF-VPRM simulations and two global CO<sub>2</sub> transport models with coastal tower measurements of CO<sub>2</sub>

“…However one of the scientific challenges is understanding the mechanisms responsible for removing anthropogenic CO 2 from the atmosphere. Observations demonstrate that about half of the emitted CO 2 is absorbed by biospheric sinks -the terrestrial biosphere and the ocean (Hansen et al, 2007). There are a number of essential questions related to the biogeochemical cycle of CO 2 to be solved by the scientific community.…”