Hypothesis 2a: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first- order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades.Why did they pick this one over the others?
Hypotheses 2a and 2b are two different oppositional views to hypothesis 1. Hypotheses 2a and 2b both agree that human impacts on climate variations and changes are significant. They differ, however, with respect to which human climate forcings are important. Because hypothesis 1 is not well supported, our scientific view is that human impacts do play a significant role within the climate system. Further, we suggest that the evidence in the peer- reviewed literature (e.g., as summarized by National Research Council (NRC) ) is predominantly in support of hypothesis 2a, in that a diverse range of first- order human climate forcings have been identified.What are these other climate forcings?
In addition to greenhouse gas emissions, other first- order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008], the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer.Why does it matter?
Because hypothesis 2a is the one best supported by the evidence, policies focused on controlling the emissions of greenhouse gases must necessarily be supported by complementary policies focused on other first- order climate forcings. The issues that society faces related to these other forcings include the increasing demands of the human population, urbanization, changes in the natural landscape and land management, long- term weather variability and change, animal and insect dynamics, industrial and vehicular emissions, and so forth. All of these issues interact with and feed back upon each other. The impact on water quality and water quantity, for example, is a critically important societal concern. The water cycle is among the most significant components of the climate system and involves, for example, cloud radiation, ice albedo, and land use feedbacks [NRC, 2003]. Regional and local variations in water availability, water quality, and hydrologic extremes (floods and droughts) affect humans most directly.Surely the IPCC is on top of this? The authors suggest not:
If communities are to become more resilient to the entire spectrum of possible environmental and social variability and change [Vörösmarty et al., 2000], scientists must properly assess the vulnerabilities and risks associated with the choices made by modern society and anticipate the demands for resources several decades into the future. Moreover, since the climate, as a complex nonlinear system, is subject to abrupt changes and driven by competing positive and negative feedbacks with largely unknown thresholds [Rial et al., 2004], scientists’ ability to make skillful multidecadal climate predictions becomes much more complicated, if not impractical.
The evidence predominantly suggests that humans are significantly altering the global environment, and thus climate, in a variety of diverse ways beyond the effects of human emissions of greenhouse gases, including CO2. Unfortunately, the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment did not sufficiently acknowledge the importance of these other human climate forcings in altering regional and global climate and their effects on predictability at the regional scale. It also placed too much emphasis on average global forcing from a limited set of human climate forcings. Further, it devised a mitigation strategy based on global model predictions. For example, although aerosols were considered as a global average forcing, their local effects were neglected (e.g., biomass burning, dust from land use/land cover management and change, soot from inefficient combustion).
Because global climate models do not accurately simulate (or even include) several of these other first- order human climate forcings, policy makers must be made aware of the inability of the current generation of models to accurately forecast regional climate risks to resources on multidecadal time scales. For example, how the water cycle responds to the diversity of climate forcings at the regional level will be important information to policy makers seeking to mitigate risks to water resources.
We recommend that the next assessment phase of the IPCC (and other such assessments) broaden its perspective to include all of the human climate forcings. It should also adopt a complementary and precautionary resource- based assessment of the vulnerability of critical resources (those affecting water, food, energy, and human and ecosystem health) to environmental variability and change of all types. This should include, but not be limited to, the effects due to all of the natural and human causedclimate variations and changes.