Donald F. Neidig

January, 2008

The rise in global mean temperature over the last 200 years is unprecedented in Earth’s recent history, and has no parallel during the 10,000 years of the Holocene. This conclusion and the data supporting it encompass what is perhaps the most carefully scrutinized series of scientific studies in modern times. The momentous nature of these studies prompted the U.S. Congress to task the National Academy of Sciences to independently review and assess the data, methods, and publications relating to global temperature. The Academy presented its findings in a review (2006) stating, "It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20^th century than during any comparable period during the preceding four centuries."

- Global temperature change is based on multiple lines of evidence: direct instrumental measurements (surface, atmospheric, satellite), numerous calibrated temperature proxies, as well as indicators in nature.

It’s the greenhouse effect

Evidence strongly indicates that the recent rise in global temperature is caused by an enhanced greenhouse effect. Quantitative agreement exists between observed increase in global temperature and the climate forcing that is calculated from the known increases in greenhouse gases. In addition, specific patterns of warming indicate an enhanced greenhouse effect (reduction in diurnal and seasonal temperature ranges, changes in geographical and altitudinal distributions of warming).

- Although the sun is the only significant source of heat at Earth’s surface, variations in solar output are too small to explain the rapid rise in temperature in the 20^th century. Patterns of warming predicted by changes in solar output are not in agreement with observed patterns. However, solar variability was likely a factor in the smaller temperature changes that occurred in the several hundred years prior to the 20^th century.

- Atmospheric aerosols produced by volcanoes and by human industry generally have a cooling effect on climate, but the warming due to greenhouses gases has outweighed the effects of aerosols.

Concentrations of greenhouse gases have risen well above their pre-industrial levels, and these increases have occurred in step with industrial activity. The sources of the added levels of the principal greenhouse gases (carbon dioxide, methane, nitrous oxide, halons, and tropospheric ozone) have been positively identified and quantitatively linked to human activities.

- Water vapor is the strongest contributor to Earth’s greenhouse effect, yet carbon dioxide remains the most important gas in modulating the total greenhouse effect. This is because atmospheric water vapor is not emission dependent but, rather, is temperature dependent; thus it responds as a feedback mechanism due to evaporation following warming by carbon dioxide and other greenhouse gases whose equilibrium concentrations do not depend on temperature (at least not yet!).

Climate warming following an ice age is a phenomenon different from the current global warming. The prevailing view is that the timings of ice ages are paced by small variations and hemispheric asymmetries in solar heating caused by Earth orbital/axial alignments. These relatively small changes then trigger much stronger climate forcings owing to feedback mechanisms within the climate system. Ice ages demonstrate that climate should not be assumed to be robustly stable in the presence of external forcing. The astronomical circumstances that would favor the onset of another ice age are not likely to recur for thousands of years. But scientific understanding of ice ages is incomplete.

Prediction of future climate is probably a more manageable problem than weather prediction which is subject to short-term chaos. Future climate is dependent upon greenhouse gas and aerosol emissions as well as future changes in solar and volcanic activity.

- Climate predictions are limited by the accuracy of predictions for future greenhouse gas emissions. Future trends in greenhouse gases do not derive from climate science, but rely on socio-economic models. A continued upward trend in emissions seems inevitable throughout most of the 21^st century.

- For any given future scenario of greenhouse gas emissions, climate models are further subject to limitations of their own.

- A "medium" climate scenario for Year 2100 indicates global mean temperature about 4 F higher relative to Year 1990 (5 F higher in the continental U.S. and 9 F higher in the arctic region).

The impacts of climate change are not limited solely to temperature, but include other variables such as sea level, drought, and flooding. A seemingly innocuous few degrees of global temperature change translates to far greater changes near the tail of the distributions of these other variables. The result is a disproportionately larger shift in the frequency of extreme weather events, the effects of which are not expected to be geographically uniform.

- The 0.2 m rise in sea level over the past century has been due mostly to thermal expansion of sea water, not melt runoff. An additional sea level rise of 0.4 m for a "medium" scenario (or 0.7 m for a "high" scenario) is projected by 2100. Sea level rise up to 6 m due to melting of the Greenland ice sheet is conceivable, but would require sustained warm temperatures for perhaps 1000 years.

- Both drought and flooding result from intensification of the hydrologic cycle due to increased atmospheric water vapor content and warmer temperatures. Precipitation globally is expected to continue to increase. Ironically, dry regions are expected to become drier, wet regions wetter.

- Cited impacts: coastal erosion, property loss, flooding, population displacements, health issues, border issues, effects on agriculture, food and water supplies, economic and political stress.

The primary objective of mitigation is to reduce greenhouse gas emissions. A strategic plan for mitigation must include awareness of the cumulative nature of CO₂ and the warming it produces: (1) atmospheric residence time of a given addition of CO₂ is a few centuries, (2) the temperature equilibrium time scale for any given level of CO₂ is also a few centuries (due to the large thermal inertia of the climate system, primarily the ocean), (3) the magnitude of climate forcing is dependent on the concentration of greenhouse gases at any given time, rather than their rate of production. Thus, if emissions were reduced to zero, warming would continue for a long time; if emissions are not curbed the warming effect will be even greater. Mitigation efforts, therefore, should be directed toward reducing or capturing greenhouse gases before further atmospheric accumulations can occur.

- World energy efficiency is improving, but rising population and energy consumption have resulted in a net increase in greenhouse gas emission rates of 70% in the last 35 years.

- A stabilization level of all greenhouse gases at about 600-700 parts per million of CO₂-equivalent will likely be encountered sometime in the latter half of the 21^st century even if mitigation efforts are undertaken. For comparison, the current level of CO₂ is 380 ppm, and the current CO₂-equivalent of all greenhouse gases combined is 455 ppm.

- A tolerable stabilization level of greenhouse gases is likely achievable through a combination of conservation, adaptation, carbon emissions reduction and/or carbon capture and sequestration, and possibly geo-engineering solutions (the latter are presently considered risky and speculative).

- Mitigation will be easier and more effective the sooner it begins, before additional gases are allowed to accumulate and before additional carbon-intensive infrastructures are put in place.

- The cost of mitigation has been studied using socio-economic models, resulting in a wide range of estimates. In comparison to a non-mitigation baseline that includes the cost of continued climate change, a "typical" mitigation scenario might cost about 0.1% per year of global GDP over the next 50 years.