Climate change is affecting the health of millions of people through altering the geographic distribution and incidence of climate-sensitive health outcomes, including through injuries, illnesses, and deaths due to extreme weather events, food-, water-, and vectorborne diseases, air pollution, aeroallergens, and malnutrition (1). The magnitude of impacts is projected to increase as the climate continues to change. The scope and scale of projected impacts mean that climate change will touch the professional and personal lives of many public health and health care professionals. Avoiding, preparing for, and effectively responding to the health risks of climate change will require broad engagement of scientists, decision-makers, and the public. Achieving this engagement means that increased scientific literacy is needed of the causes and implications of climate change for human health. There are growing numbers of assessments of the potential health impacts of climate change (1–4) and summaries for various audiences (5). Instead of summarizing current knowledge of the causes and consequences for human health of anthropogenic climate change, this paper will highlight a few issues that may help with understanding the human health impacts of climate change. Energy drives climate change as well as its consequences Energy refers to both the energy derived from burning fossil fuels, which, along with deforestation, is the primary cause of anthropogenic climate change, and to the resulting increased energy within the climate system that is manifest in the form of increased temperatures, alterations of the hydrologic cycle, and more frequent and intense extreme weather events. The principal driving force for weather and climate is the uneven warming of the Earth’s surface (due to the angle of rotation). Complex and changing atmospheric and oceanic patterns redistribute the absorbed solar energy from the equator to the poles. In addition, some absorbed energy is reradiated as long-wave (infrared) radiation. Some of this infrared radiation is then absorbed by the atmospheric greenhouse gases (including water vapour, carbon dioxide, methane, nitrous oxide, halocarbons, and ozone) and reradiated back to the Earth, thereby adding additional energy to the atmosphere and oceans. This greenhouse effect warms the surface by more than would be achieved by incoming solar radiation alone and raises the global average surface temperature to its current 15°C (6). Without this warming, the Earth’s diurnal temperature range would increase dramatically, and the global average surface temperature would be about 33°C colder. Increasing concentrations of greenhouse gases are increasing the energy in the atmosphere, further warming the planet. Current concentrations of atmospheric CO2 and ethane far exceed pre-industrial values found in polar ice core records dating back 650,000 years; the concentration of atmospheric CO2 has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005 (7). Since 1750, it is estimated that about 2/3rd of anthropogenic CO2 emissions have come from fossil fuel burning and about 1/3rd from land use change. Ambient temperatures increased 0.74°C worldwide over the period 1906–2005. The rate of warming averaged over the past 50 years (0.13°C + 0.03°C per decade) is nearly twice that for the last 100 years (Figure 1), and at least six times faster than at any time during the 2,000 years. Figure 1: Figure 1: Global mean temperature increase (°C) from 1850 to 2005 (7). Annual global mean temperatures (black dots) with linear fits to the data. The left axis shows temperature anomalies relative to 1961–1990 and the right hand axis shows ... The Earth is committed to decades of climate change Figure 2, from the Intergovernmental Panel on Climate Change (IPCC), shows projected temperature changes over this century based on different scenarios of greenhouse gas emissions (7). These scenarios are based on differing assumptions of future demographic, economic, and technology changes that determine the amount of fossil fuels that will be burned. The IPCC projected that the global mean temperature of the Earth would increase by the end of the 21st century by between 1.1 and 6.4°C (7). This projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years. Figure 2: Observed and Projected Global Mean Surface Temperatures (°C), 1900 to 2100. Solid lines are multi-model global averages of surface warming (relative to 1980–1999) for the scenarios A2, A1B and B1, shown as continuations of the 20th century ... The orange line on the figure is the climate change commitment or the amount of warming that will occur no matter the degree to which greenhouse gas emissions are reduced over the next few decades. This commitment is due to the inherent inertia in the climate system. Natural processes currently remove about half the incremental anthropogenic CO2 added to the atmosphere annually; the balance is removed over one to two hundred years (7). The Earth is committed to nearly as much warming as has already occurred.