Stephen H. Schneider

Integrated studies of coupled human and natural systems reveal new and complex patterns and processes not evident when studied by social or natural scientists separately. Synthesis of six case studies from around the world shows that couplings between human and natural systems vary across space, time, and organizational units. They also exhibit nonlinear dynamics with thresholds, reciprocal feedback loops, time lags, resilience, heterogeneity, and surprises. Furthermore, past couplings have legacy effects on present conditions and future possibilities.

Nearly 20 years ago, it was suggested that individuals exist who are not obese on the basis of height and weight, but who, like people with overt obesity, are hyperinsulinemic, insulin-resistant, and predisposed to type 2 diabetes, hypertriglyceridemia, and premature coronary heart disease. Since then it has become increasingly clear that such metabolically obese, normal-weight (MONW) individuals are very common in the general population and that they probably represent one end of the spectrum of people with the insulin resistance syndrome. Available evidence also suggests that MONW individuals could account for the higher prevalence of type 2 diabetes, cardiovascular disease, and other disorders in people with a BMI in the 20-27 kg/m2 range who have gained modest amounts of weight (2-10 kg of adipose mass) in adult life. Specific factors that appear to predispose MONW, as well as more obese individuals, to insulin resistance include central fat distribution, inactivity, and a low VO2max. Because these factors are potentially reversible and because insulin resistance may contribute to the pathogenesis of many diseases, it is our premise that a compelling argument can be made for identifying MONW individuals and treating them with diet, exercise, and possibly pharmacological agents before these diseases become overt, or at least early after their onset. One reason for doing so is that disorders such as type 2 diabetes may be accompanied by irreversible consequences, e.g., ischemic heart disease and nephropathy, at the time of diagnosis or shortly thereafter. Another is that MONW individuals in general should be younger and more amenable and responsive to diet and exercise therapy than are obese patients with established disease. That long-term diet and exercise can work is suggested by two large studies in which, over 5-6 years, the incidence of diabetes was diminished in nonobese and minimally obese patients with impaired glucose tolerance. Based on these considerations and the emerging worldwide epidemic of type 2 diabetes, we believe that studies to assess whether therapies aimed at young MONW individuals can prevent the development of type 2 diabetes and other diseases, including perhaps obesity itself, are urgently needed.

The magnitude of future climate change depends substantially on the greenhouse gas emission pathways we choose. Here we explore the implications of the highest and lowest Intergovernmental Panel on Climate Change emissions pathways for climate change and associated impacts in California. Based on climate projections from two state-of-the-art climate models with low and medium sensitivity (Parallel Climate Model and Hadley Centre Climate Model, version 3, respectively), we find that annual temperature increases nearly double from the lower B1 to the higher A1fi emissions scenario before 2100. Three of four simulations also show greater increases in summer temperatures as compared with winter. Extreme heat and the associated impacts on a range of temperature-sensitive sectors are substantially greater under the higher emissions scenario, with some interscenario differences apparent before midcentury. By the end of the century under the B1 scenario, heatwaves and extreme heat in Los Angeles quadruple in frequency while heat-related mortality increases two to three times; alpine/subalpine forests are reduced by 50-75%; and Sierra snowpack is reduced 30-70%. Under A1fi, heatwaves in Los Angeles are six to eight times more frequent, with heat-related excess mortality increasing five to seven times; alpine/subalpine forests are reduced by 75-90%; and snowpack declines 73-90%, with cascading impacts on runoff and streamflow that, combined with projected modest declines in winter precipitation, could fundamentally disrupt California's water rights system. Although interscenario differences in climate impacts and costs of adaptation emerge mainly in the second half of the century, they are strongly dependent on emissions from preceding decades.

Humans have continuously interacted with natural systems, resulting in the formation and development of coupled human and natural systems (CHANS). Recent studies reveal the complexity of organizational, spatial, and temporal couplings of CHANS. These couplings have evolved from direct to more indirect interactions, from adjacent to more distant linkages, from local to global scales, and from simple to complex patterns and processes. Untangling complexities, such as reciprocal effects and emergent properties, can lead to novel scientific discoveries and is essential to developing effective policies for ecological and socioeconomic sustainability. Opportunities for truly integrating various disciplines are emerging to address fundamental questions about CHANS and meet society's unprecedented challenges.