Climate change is a regular topic of discussion on Geekzone. It seems that climate change discussions always degenerate into a stalemate, with no views changed on any side of the discussion. I think the outcome is, in part, due to people holding different "facts" about the topic.
Therefore, I thought it might be interesting to state my current understanding of the science. I've studied climate science, as an amateur, for 25 years. Climate science has evolved greatly over that time. There is still much that we don't understand, especially about the feedback loops in the system, but there is also a lot that is well established. This understanding is, of course, subject to change given new evidence or correction of errors.
I don't expect to change anyone's views, but maybe the discussions can be a little more informed.
TL;DR: Earth's climate is warming. Humans have contributed by emitting greenhouse gases. The future path is uncertain, but it is likely to be further warming.
The greenhouse effect:
- Some atmospheric gases transmit visible and UV light from the Sun but absorb infrared light (directly from the Sun and reflected by the surface) - this is the "greenhouse effect", since the gases work much like the glass in a greenhouse.
- Without the greenhouse effect, Earth's average surface temperature would be about -18°C. That is, 33°C colder than the current average temperature of 15°C.
- The vast majority of life on Earth depends on the greenhouse effect to create a tolerable climate.
- By far the most significant greenhouse gas is water vapour, though CO2, methane, and some other gases also contribute.
- Human activity has little direct impact on the amount of water vapour in the atmosphere.
- Feedback effects due to temperature change may be important, but this is uncertain.
- For example, clouds reflect sunlight back into space, but clouds also trap heat in the atmosphere. The net effect is uncertain.
- Since the start of the industrial revolution, 250 years ago, humans have emitted 2 trillion tonnes of CO2.
- Around half of the emissions have been since 1975.
- We currently emit about 35 billion tonnes of CO2 per annum (or about 10 billion tonnes of carbon per annum - the two number are often confused).
- For 800,000 years prior to the industrial revolution, CO2 concentration in the atmosphere was in the range 170 to 300 parts per million (ppm) with an average of 240 ppm.
- For 2,000 years prior to the industrial revolution, CO2 concentration in the atmosphere was in the range 270 to 290 ppm.
- Global average CO2 concentration in the atmosphere is now 405 parts per million and rising at 2 ppm per year.
- CO2 has an atmospheric half-life of decades (subject to a large range of uncertainty, extending to hundreds of years according to some estimates).
- The long life of CO2 in the atmosphere is the main reason why it is important.
- All the rise in CO2 concentration is accounted for by human-related emissions of CO2.
- Methane is many times more potent than CO2, in terms of the greenhouse effect.
- The methane concentration has risen from 700 to 2000 ppm since the start of the industrial revolution.
- Methane cycles out of the atmosphere fairly quickly, with an atmospheric half-life of 7 years.
- The short life of methane makes it less important than its potency would otherwise suggest.
- Unhelpfully, some atmospheric methane oxidises to form atmospheric CO2.
Average global atmospheric temperature:
- Over the last 500 million years Earth's average atmospheric temperature has varied by about +/- 10°C, due to variations in three main factors: the Sun, Earth's orbit, and atmospheric composition.
- From about 20,000 to 10,000 years ago, the climate warmed by about 5 to 7°C as the planet emerged from the last ice age.
- From 10,000 years ago until around the year 1900, the global average temperature varied within +/- 0.5°C almost all the time. This period coincides with the development of agriculture, domestication of animals, and the rise of civilisations.
- Since 1900, average global temperature has risen by almost 1°C. This rise is consistent with the size of the greenhouse effect expected from the higher CO2 (and methane, etc.) concentration.
- Most of the rise in temperature has occurred since 1980.
- Sea level has varied radically over time, with a range of several hundred metres.
- Sea level rose by more than 100m (5 to 15mm/year) over the period from 15,000 years ago to 7,000 years ago, as the planet emerged from the last ice age.
- Sea level continued rising from 7,000 to 2,000 years ago, at around 1 mm/year.
- Over the 2,000 years before the industrial revolution, there was no significant change in average sea level.
- Over the last few decades, average sea level has risen by 2-4mm/year (with a lot of variation between locations), with an increasing rate.
- Over the long term, sea level change is driven mostly by the amount of ice on land (primarily Antarctica and continental ice sheets).
- Over the last 100 years, sea levels have risen by about 200mm.
- The recent rise in sea level is more-or-less evenly due to thermal expansion of water as the oceans have absorbed extra heat from the atmosphere and extra water due to melting land ice.
- If the Antarctic ice sheet melts, then it would raise sea levels by 60m - though that would take at least hundreds of years and more likely thousands of years.
- Earth's orbit varies, with short and long (100,000+ years) cycles.
- Variations in the orbit produce variations in sunlight intensity, which is the primary cause of the ice age cycle.
- The variations are fairly well understood and occur too slowly to account for any significant part of the recent warming.
- The Sun provides almost all heat on Earth (the rest comes from gravitation compression of the Earth, drag from the Moon, radioactive decay, etc).
- Without the Sun, there would be no climate variation; the Earth would be a frozen, dead rock with a surface temperature of around -250°C to -210°C.
- The Sun's energy output varies over time.
- The size of the variation over the last 100 years is tiny: 1366 +/- 1 Watts per square metre at the top of the atmosphere. Too small to materially change average temperatures.
- Atmospheric temperature is related to the sunspot cycle, rather than energy output, for reasons that are not well understood.
- The number of sunspots varies in an 11 year cycle, with some cycles being more intense than others.
- Lower temperatures are associated with fewer sunspots.
- We are currently in a cycle that has low sunspot counts, which may be offsetting some warming that would otherwise occur.
- Some analyses suggest even lower sunspot counts over the next couple of decades.
- The "Maunder minimum", also known as "The Little Ice Age", occurred during a period of especially low sunspot counts, though the effect may have been localised to Europe rather than being global.
- Forecasting climate change, and effects like sea level change, is very difficult and subject to a lot of uncertainty.
- The Sun warms the Earth by around 210°C. The natural greenhouse effect adds another 33°C. Humans have added about 1°C over the last 100 years and will likely add 1-2°C more in the next 100 years.
- Essentially, the models are trying to forecast a relatively small effect in a system that is quite variable, very complex, and inadequately understood.
- The models inevitably get the details wrong, but the general patterns are clear: more greenhouse gases imply greater heat retention and hence higher average global temperatures.
- The warming tendency may be offset by low sunspot activity - though this is very speculative.
- Almost all climate models have over-estimated the warming trend over the last 20 years. The reasons for this are not well understood.
- The main source of modelling uncertainty is the magnitude of feedback effects in the system.
- Some feedback effects reduce warming (e.g. warmer atmosphere has more clouds, and clouds reflect visible light back into space), while others increase warming (e.g. less Arctic sea ice exposes water to sunlight, and water absorbs more heat than ice does).
- Subject to potential off-setting effects, and notwithstanding the tendency to over-estimate impacts, almost all climate models predict a warming trend if greenhouse gas concentrations continue to rise.