Is the Earth’s climate changing (atmosphere warming)?
If so, is this due to human activities?
Ř Examine past variations in climate (over geological – recent times).
Ř Examine possible factors involved.
đ Greenhouse effect keeps planet surface warm
đ Abundance of ‘greenhouse gases’ is increasing
đ Past T and greenhouse gas concentrations are correlated
it seems obvious that the warming is occurring (and will continue to).
But not that simple
¨ other factors influence the energy budget.
¨ changes in radiation may lead to other changes which effect the energy budget (“feedback”).
It is important to realize that the Earth’s temperature (climate) is not just determined by atmospheric processes.
Must consider the whole “climate system” which includes the atmosphere, hydrosphere, solid Earth, and biosphere.
The climate of Earth has undergone numerous changes over a range of time scales.
Over longest time scale Earth has been mostly warm (i.e. little ice) but several `brief’ ice ages. We are currently in latest (last 5 million yrs).
Climate not uniform within this ice age, and there are large oscillations in temperature and ice cover: glacial/interglacial cycles.
We are now in a warm interglacial cycle.
Oscillations also present at smaller scales (1 – 100 thousands yrs); indicating climate system has tendency to flip between cold and warm phases.
Over last 100 yrs there have been two phases of warming (separated by period with no significant change), the second started in 1970s.
9 of warmest years have occurred in last 11 years, and 2 coolest years in 1990s occurred following Mt Pinatubo eruptions.
Note that climate changes are not restricted to changes in mean values; there are also changes in occurrence of extreme events (e.g., heavy precipitation, drought).
METHODS FOR DETERMINING PAST CLIMATE
No direct measurements of climate except in last 200 years, so need to reconstruct climate from other measurements (“proxies”)
đ glaciers (exist only in cold conditions)
đ coral reefs (past sea-level and water temperatures)
đ stratigraphy (sequencing of high and low precipitation)
đ radiocarbon dating of preserved pollen (< 50,000 yrs)
đ tree rings (~1000 yrs)
- sediments record changes in near-surface organisms
đ oxygen isotopes of calcium carbonate (linked to glaciers)
đ T from oxygen isotopes
đ Information of chemical composition (e.g., CO2, CH4)
The (very slow) movement of “slabs” in the outer portion of Earth’s crust explains not only the continents and oceans but also climate change on the longest time scales (100 millions of years).
Changes In Earth’s Orbit
Regular long-term changes in the Earth-sun geometry alter the solar forcing of the climate system:
1. Eccentricity of orbit ~100,000 yr period.
2. Tilt of Earth’s axis ~ 41,000 yr period.
3. Precession of axis ~ 23,000 yr period.
Collectively known as Milankovitch cycles.
Close link with some reconstructured climate fluctuations.
The energy from the sun is not constant, and varies with ~11yr period (sunspot cycle).
There may also be a long-term trend, but over last 200 yrs this is small.
Atmospheric particles (aerosols)
č have low albedo -> absorb insolation -> warm the atmosphere.
č stimulate cloud growth -> changes absorption and reflection
č directly affect long-wave radiation
Current opinion (models) indicate that the net effect of increased aerosols is to cause a cooling of the atmosphere.
Hence large Volcanic eruptions can strongly influence the climate.
For example, Mt Pinatubo eruption caused a 0.6C cooling for 2-3 years.
A system (such as climate system) can respond to a force in such a way as to increase / decrease the effect of original force. This is known as a positive / negative feedback.
As the climate system is complex, there are many feedback mechanisms. For example,
Changes in Ice Albedo
Warmer planet -> less ice -> lower albedo -> higher absorption -> increased warming. (Positive).
Changes in Water Vapor and Clouds
Warmer planet -> greater evaporation -> increased water vapor -> enhanced greenhouse effect -> increased warming. (Positive).
But also more water vapor -> increased cloudiness
-> more reflection -> cooling (Negative)
-> increased greenhouse effect -> warming (Positive)
Current evidence suggests net effect is cooling (Negative),
but very uncertain.
Changes in Atmosphere-Ocean Interactions
Higher T -> less CO2 dissolved -> less uptake -> increased warming. (Positive).
[NOTE: Ocean uptake of CO2 accounts for a large amount of CO2 added by humans.]
Although possible atmosphere-ocean feedbacks involve changes in ocean currents and in chemistry and biology within the oceans.
Note over geological time scales changes in the thermohaline circulation are very important.
Changes in Atmosphere-Biota Interactions.
Warmer planet -> drier surface -> reduced evaporation
-> increased warming. (Positive).
-> less low clouds -> increased warming. (Positive).
Warmer planet -> increased photosynthesis & vegetation -> increased suppression of greenhouse gases -> less warming. (Negative).
As complex system need models to predict future.
Current climate models predict a global warming of 1.5-3.5 OC from double CO2.
But considerable uncertainty.
· don’t adequately simulate the role of oceans
· can’t resolve local/regional scales (esp. clouds)
· limited by chaotic nature of climate system.