Human activity is disrupting Earth’s climate, and the rising emissions of greenhouse gases are accelerating that disruption. (See “Our Changing Climate” box below) Some of the health consequences of climate change are straightforward: warmer temperatures, changes in the hydrologic cycle, increased ground-level ozone, and enhanced pollen production will increase exposure to heat stress, alter patterns of infectious disease, and compromise air quality. These and similar direct impacts of climate change have been well covered in the scientific literature, and what we know of their likely considerable effects is discussed in the sidebar “Direct Impacts of Climate Change” below. Climate Change and the Biosphere

However, we believe that there is another threat, one that is orders of magnitude more potent than those which have been emphasized to date. Here we argue that it is the indirect impacts of climate change-large-scale alterations to Earth’s natural systems-that pose the greatest risk to human health. These changes are curtailing access to water and to food and are undermining the very concept of stable homes, yet have received scant attention in the literature, including the report of Working Group II of the Intergovernmental Panel on Climate Change (IPCC).1 Enormous uncertainties surround predictions regarding how climate change may affect human well-being. Acknowledging these uncertainties is a critical component of designing optimal approaches to mediating the health impacts of climate change. Water

Water scarcity is already a major global issue that carries heavy adverse health sequelae, and climate change will further destabilize access to fresh water.