as methane or nitrous oxide are significant for climate change in the next few decades or century, but these gases do not persist over time in the same way as carbon dioxide'' (Solomon et al. 2009(Solomon et al. , p 1705)).
Reasons for Concern
Reasons for concern about stabilizing greenhouse gases in the atmosphere that impact climate change are identified in the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (Smith et al. 2009). The risks after 1990 for each increase in temperature from 0 to 5 8C in global mean temperature are substantial: 1) risk to unique and threatened systems, 2) risk of extreme weather events, 3) distribution of impacts, 4) aggregate damages, and ( 5) risks of large-scale discontinuities (e.g., tipping points). Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) commits signatory nations to stabilizing greenhouse gas concentrations in the atmosphere at a level that ''would prevent dangerous anthropogenic interference (DAI) with the climate system'' (United Nations 1992; http://unfcc.int/resource/docs/convkp/convegn.pdf). ''The UNFCCC also highlights 3 broad metrics with which decision-makers are to assess the pace of progress toward this goal: allow 'ecosystems to adapt naturally to climate change,' ensure that 'food production is not threatened,' and enable 'economic development to proceed in a sustainable manner''' (Smith et al. 2009, p 4133).
The above metrics are very difficult assessment and management goals, especially because the present reasons for concern indicate that the perceived risks are significantly greater than they were in 2001. The changed perception of risks is quite understandable because much more data were available in 2009 than in 2001, but the methods and procedures for assessing risks during rapid climate change are, at best, in the early formative stages.
''As was true in the TAR (Third Assessment Report of the IPCC), the aggregation of risk across many different sectors, regions, or populations under a particular reason for concern is subjective and thereby introduces another source of uncertainty'' (Smith et al. 2009, p 4135). However, anthropogenic greenhouse gas emissions are still increasing substantially, and, as global mean temperature increases, carbon emissions in positive feedback loops from stored carbon (e.g., wetlands and tundra) are likely to increase as well, further exacerbating the problem.