Global Warming The name given to the increase in the average temperature of the Earth’s near surface temperature and oceans that has occurred since the middle of the 20 th century. In the graph to the right, panel a gives the Earth's surface temperature is shown year by year (red bars) and approximately decade by decade (black line, panel b gives the year by year (blue curve) and 50 year average (black curve) variations of the average surface temperature of the Northern Hemisphere for the past 1000 years have been reconstructed from "proxy" data calibrated against thermometer data.

Greenhouse effect Main culprit behind global warming Sunlight enters the atmosphere and warms the Earth (UV, visible, IR) At night, Earth radiates heat energy it gained during the day(mostly at IR wavelengths) Not all of this energy escapes the atmosphere-some of it is absorbed and re-emitted back into the atmosphere, warming the atmosphere and the Earth. Not necessarily a bad thing- without it the Earth would be cold and life (a least as we know it) would not be possible

Venus Hottest planet in the solar system But not the closest to the sun Atmosphere is 96.5% carbon dioxide Venus once had immense amounts of water, but no ozone layer-the UV radiation broke up the water molecules Unfortunate, water absorbs carbon dioxide wand mitigates the greenhouse effect Resulted in an extreme greenhouse effect

Greenhouse effect-What could happen Global temperatures rise until boiling point of water is reached As water boils away, it becomes water vapor in the atmosphere, accelerating the heat rise (called positive feedback). At a few hundred degrees celsius, carbon dioxide would be released from the rocks (sublimation), accelerating the heating again Eventually the Earth would stabilize at surface temperatures similar to Venus (T=860F)

Greenhouse Gasses –in order of importance water vapor carbon dioxide methane nitrous oxide ozone CFCs

Changes in greenhouse gas concentrations Historical variations can be tracked via analysis of ice cores – Ice core - a core sample (a cylindrical section of a naturally occurring medium consistent enough to hold a layered structure) from the accumulation of snow and ice over many years that have re- crystallized and have trapped air bubbles from previous time periods. – The composition of these ice cores provides a picture of the climate at the time. – Record for over 800,000 years Gas Preindustrial Level Current Level Increase since 1750 Carbon dioxide 280 ppm 387ppm 104 ppm Methane 700 ppb 1,745 ppb 1,045 ppb Nitrous oxide 270 ppb 314 ppb 44 ppb CFC ppt 533 ppt

What do the ice cores tell us? Natural variations occur in the greenhouse gas concentrations

Post industrial revolution

Man made sources of greenhouse gasses Also called Anthropogenic, which designates an effect or object resulting from human activity burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations. – Account for one third of total anthropogenic CO2 emissions. livestock enteric fermentation and manure management, paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane. use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes. agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide (N2O) concentrations.

What are we seeing? Since , global surface temperatures have increased by 1.35 F Urban heat island effect only accounts for 0.02 C rise Since 1979 land T have increased twice as fast as ocean Temperatures Temperatures on the lower troposphere have have increased by 0.62 between 1979 and 2000.

Hold on, could it be the sun? No overall increase in solar brightness in over 1000 years Solar cycles do cause small variations in brightness, but not enough to account for what has been seen No its not the sun

Feedback-making the problem worse (or better) Positive feedback-when the warming induces further warming Negative feedback – when the warming induces a cooling

Sources of Feed back Positive – Water vapor – Clouds – Ice albedo – Arctic methane release – Reduced carbon dioxide absorption in the oceans Negative – Lapse rate

Negative feedback Lapse Rate Increased heating means increased IR emission Global warming reduces the rate of Temperature decrease with height, which means more long wavelength radiation will be emitted by the upper atmosphere This will weaken the greenhouse effect

Positive feedback Water vapor – Warming increases the amount of water vapor in the atmosphere, which increases the warming since water vapor is also a greenhouse gas Clouds – Act as a blanket, reflect IR radiation downward toward the surface Ice albedo – Melting ice reveals land and water, both reflect less light than ice- more warmth is absorbed increasing warming Arctic methane release – Warming releases sources of carbon dioxide Reduce carbon dioxide absorption in oceans – Warm waters favor the growth of plankton rather than diatoms- diatoms are more efficient carbon dioxide absorbers

Global dimming An effect that has been counteracting some of global warming from about 1960 forward Aerosols produced by volcanoes and pollutants such as sulfur dioxide reflect incoming sunlight Soot –suspended in the atmosphere, it can absorb solar radiation and heat the atmosphere, but cool the surface

What are we seeing: Besides the increases in Temperature: – World’s glaciers are melting – Arctic sea ice is reducing in both extent ( 9% reduction in area per decade) and thickness (15-40% in thickness in the last 30 years) – Ocean levels are rising-both due to melting of Antarctic ice and thermal expansion of sea water – Longer growing seasons – Thawing of permafrost in Alaska – Coral reef bleaching-whitening of reefs due to increased temperature – Earlier plant flowering – Earlier bird arrivals – Shifting of animal ranges poleward – More frequent EL Nino-warming of the Pacific ocean surface temperatures-causes changes in local weather patterns

Global warming-the debate The fact that the Earth is warming is not a matter of debate, the evidence is clear The fact that global carbon dioxide levels have increased is also clear The debate centers around how much of it is caused by human involvement vs a natural cycle Think about it in terms of the steps in the scientific method:

Global warming in the context of the scientific method Observations-Earth’s temperature is warming Hypothesis-due to a man-made increase in green house gases Testing the hypothesis-many scientists hold different views and interpret data differently, but the consensus is that it is due to increases in greenhouse gasses from man made sources. Until the data is incontrovertible, there will always be naysayers. That’s ok, they keep us honest and push the method forward Lots of examples like this in modern science (evolution, big bang, cosmological interpretations of galaxy redshifts)