1. There is a significant atmospheric greenhouse effect on Earth that results in the surface temperature being much warmer than it would be without the atmosphere. This greenhouse effect is mainly the result of greenhouse gases.
2. The atmospheric concentrations of several greenhouse gases are increasing due to human activities.ï¿½ The most important of which is carbon dioxide (CO2), which has increased from 280 ppm to over 400 ppm.
3. There has been a measurable increase in global average surface temperature of about 0.9ï¿½C (1.6ï¿½F) since 1860. Note that facts 1 ï¿½ 3 are not proof that the increase in greenhouse gases has been largely responsible for the increase in temperature.
4. The climate of Earth has changed all through its history regardless of human activity.
1. We expect that adding greenhouse gases to the atmosphere will cause the global average temperature to increase by enhancing the natural atmospheric greenhouse effect. However, the details of how much warming and the pattern of related climate changes are uncertain. This is due to our limited understanding of the climate system and limited ability to simulate the climate system using computer models.
2. Current levels of CO2 in the atmosphere are probably higher now than at any time over the several hundred thousand years, but certainly much lower than CO2 levels were millions of years ago. Again this is not proof that the recent warming has been caused by human emissions of greenhouse gases.
3. It is likely that the global average temperature was warmer during the Holocene Optimum some 6 to 10 thousand years ago than it is today, which means the current global average temperature is not the warmest time since the end of the last ice age.
4. There is evidence in the climate record showing that rather large climate shifts have occurred over relatively short periods of time (within decades). Thus, if greenhouse gas emissions are significant drivers of climate change, then rapid changes are possible.
Questions / Uncertainties (Partial List)
1. Complete knowledge of the chemical cycles of greenhouse gases (sources/sinks) is lacking. This makes it difficult to predict current and future atmospheric concentrations of these gases. An example is the missing CO2 sink.
2. Models used to predict possible anthropogenic global warming are uncertain
ï¿½ Some feedbacks within the climate system are not well understood or properly taken into account within the models
ï¿½ Models are unable to reproduce the known regional scale variability in climate zones, which limits confidence in their ability to properly simulate changes in climate
3. Impact Studies
ï¿½ The magnitude of regional climate change and the rate at which it occurs must be compared with the sensitivity and adaptability of human populations and ecosystems.
ï¿½ Sensitivity and adaptability are uncertain even if regional climate changes were known.
4. Implication of recent warming of global average temperature
ï¿½ Has there already been some global warming due to increased greenhouse gases or is the recent warming part of a natural cycle of climate?
ï¿½ We may not be able to definitively answer this question for some time.
5. Surprises (Many more could be listed here)
ï¿½ Humans are artificially perturbing climate by adding greenhouse gases. This can be considered a grand experiment since we are not able to accurately predict the response of the climate system.
o So far changes (if they have been caused by greenhouse gas increases) have been small. Is there a danger that the relatively stable climate we now enjoy can shift unpredictably to another state if we somehow push the climate system too far from its pre-industrial state? Will it then be too late to go back? Assuming that we are measuring anthropogenic global warming already, will temperatures continue to rise slowly or will temperatures start to increase more rapidly at some point?