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Evidence of Global Warming

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Tools Scientist Use to Study Climate Change• Weather stations, buoys, satellites,

radar, ice and ocean cores, tree rings, cave deposits, native knowledge

H Graham BSc PGCE 3

Evidence of Global Warming• Carbon Dioxide Increasing in Atmosphere• Increased Temperatures• Disappearing Glaciers / Melting Arctic Sea

Ice • Rising Sea Levels• More Frequent Extreme Weather

H Graham BSc PGCE 4

Carbon Dioxide Increasing in Atmosphere • The atmospheric levels of the

greenhouse gas carbon dioxide, have increased since pre-industrial times from 280 part per million (ppm) to 400 ppm, a 41% increase.

• Carbon dioxide concentrations in the atmosphere are the highest in 160,000 years.

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H Graham BSc PGCE 6

The “Greenhouse Effect”

The Earth’s surface thus receives energy from two sources: the sun & the atmosphere – As a result the Earth’s surface is ~33C (94 F) warmer than it

would be without an atmosphere

Greenhouse gases are transparent to shortwave but absorb longwave radiation – Thus the atmosphere stores energy

The Sun’s energy passes through the car’s windshield.

This energy (heat) is trapped inside the car and cannot pass back through the windshield, causing the inside of the car to warm up.

Example of the Greenhouse Effect

http://www.youtube.com/watch?v=Hi3ERes0h84

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Selected Greenhouse Gases• Carbon Dioxide (CO2)

– Source: Fossil fuel burning, deforestation Anthropogenic (man made) increase: 30% Average atmospheric residence time: 500 years

Methane (CH4) – Source: Rice cultivation, cattle & sheep ranching, decay

from landfills, mining Anthropogenic increase: 145% Average atmospheric residence time: 7-10 years

Nitrous oxide (NO2) – Source: Industry and agriculture (fertilizers) Anthropogenic increase: 15% Average atmospheric residence time: 140-190 years

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1840 1860 1880 1900 1920 1940 1960 1980 2000

14.5

1920

14.4

14.3

14.2

14.1

14.0

13.9

13.8

13.7

13.6

13.5

13.4

58.0

57.8

57.6

57.4

57.2

57.0

56.8

56.6

56.4

56.2

www.gcrio.org/ipcc/qa/cover.html (modified)

Variability

Average

Climate Change vs. Variability

Climate Change vs. Variability

Even in a stable climate regime, there will always be some variation (wet/dry years, warm/cold years) A year with completely “average” or “normal” climate conditions is rare

Climate variability is natural.

The challenge for scientists is to determine whether any increase/decrease in precipitation, temperature, frequency of storms, sea level, etc. is due to climate variability or climate change.

The degree of warming will not be uniform everywherehigher latitudes are more sensitive

Source: IPCC

H Graham BSc PGCE 15

Disappearing Glaciers • Ice is melting all over the planet. Glaciers are

melting on six continents.  If present warming trends continue, all glaciers in Glacier National Park could be gone by 2030.

• Because of global warming, the glaciers of the Ruwenzori range in Uganda are in massive retreat.  The Bering Glacier, North America's largest glacier, has lost 7 miles of its length, while losing 20-25% of parts of the glacier. 

• The melting is accelerating. The Lewis Glacier on Mt. Kenya (In Kenya) has lost 40% of its mass during the period 1963-1987 or at a much faster clip than during 1899-1963.

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Ice shelves of the South Pole have partly separated and are collapsing. (NASA)

Receding high mountain glaciers

Melting ice and rising sea level

H Graham BSc PGCE 18

Melting Arctic Sea Ice • According to a report by

Norwegian scientists, the arctic sea ice in about 50 years could disappear entirely each summer. Researchers at the Nansen Environmental and Remote Sensing Center based their predictions on satellite pictures. These pictures showed that the Arctic winter icescapes decreased by 6% (a Texas-size area) during the last 20 years

Of course the physical environmental change will lead to changes in the biosphere – including our society.

H Graham BSc PGCE 20

Rising Sea Level in Florida

If temperature can change, so do other meteorological and environmental variables.

• The change in temperature may cause a change in precipitation.

• Vegetation may also change in response to temperature and precipitation changes.

• And there will be changes in the animal and human world in response to these environmental changes.

Source: NOAA

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More Frequent Extreme Weather • The potential for floods and droughts is increasing.".......

the heating from increased greenhouse gases enhances the hydrological cycle and increases the risk for stronger, longer-lasting or more intense droughts, and heavier rainfall events and flooding, even if these phenomena occur for natural reasons. Evidence, although circumstantial, is widespread across the United States

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Feedback Effect• The climate system is very complicated. A

change in one component of the system may cause changes in other components.

• Sometimes the changes in other components enhance the initial change, then we say that these changes have positive feedback to the system.

• If the changes result in the reduction of the original change, then they have negative feedback.

An example of positive feedback• When the climate becomes warmer (either

due to the increase of CO2 in the atmosphere or other unknown mechanisms), the ocean may also become warmer. A warmer ocean has lower solubility of CO2 and hence will release more CO2 into the atmosphere. This may cause the climate to become even warmer than before. Thus the dependence of solubility of CO2 on temperature has a positive feedback on the climate system.

An example of negative feedback• Consider a clear region over the ocean.

Since there is no cloud, the sun shines on the ocean surface, causing it to warm up. This makes this part of the ocean warmer than other parts and the air over it tends to rise (causing convection). As we have learned before, rising air expands and cools, causing clouds to form. The formation of clouds will block out the sun and the solar heating of the ocean surface will cease. The surface will start to cool down. Thus the cloud formation due to surface heating and convection is a negative feedback to the climate system.

Increased CO2

Higher temperature

More CO2

POSITIVE NEGATIVE

More water vapor & other

changes

Increased cloud cover

More reflected solar radiation

Lower temperature

Less water vapor

More absorbed infrared radiation

Higher temperature

More water vapor

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Some examples of criticisms• There are evidence showing

that the current temperature isn’t really that warm compared to what was two to three thousand years ago. The figure to the right shows that the temperature of Sagaso Sea fluctuates in a range of ~ 3.6°C.

• Also the “trend” depends on the data sets and the section of data you select to examine – see the lower chart. By using a different data set (here the satellite microwave sounding) and selecting a suitable section (for example, 1978-1998) you can actually show that there was a cooling, not warming.

Source: Robinson et al. (1998)

There are also evidence showing that the solar activity seems to have some influence on atmospheric temperature. But there are many questions here. Especially on how and how much.

Carbon Dioxide CycleThe mechanism by which Earth self-regulates its

temperature is called the carbon dioxide cycle, or the CO2 cycle for short.

Starting with the carbon dioxide in the atmosphere: • Volcanoes outgas CO2 into the atmosphere.• Atmospheric carbon dioxide dissolves in the oceans. • At the same time, rainfall erodes rocks on Earth’s

continents and rivers carry the eroded minerals to the oceans.

• In the oceans, the eroded minerals combine with dissolved carbon dioxide and fall to the ocean floor, making carbonate rocks such as limestone.

• Over millions of years, the conveyor belt of plate tectonics carries the carbonate rocks to subduction zones, and subduction carries them down into the mantle.

• As they are pushed deeper into the mantle, some of the subducted carbonate rock melts and releases its carbon dioxide, which then outgasses back into the atmosphere through volcanoes.

The CO2 CycleIf Earth warms up a bit, then • carbonate minerals form in the oceans at a higher

rate. • The rate at which the oceans dissolve CO2 gas

increases, pulling CO2 out of the atmosphere. • The reduced atmospheric CO2 concentration leads

to a weakened greenhouse effect that counteracts the initial warming and cools the planet back down.

If Earth cools a bit, • carbonate minerals form more slowly in the

oceans. • The rate at which the oceans dissolve CO2 gas

decreases, allowing the CO2 released by volcanism to build back up in the atmosphere.

• The increased CO2 concentration strengthens the greenhouse effect and warms the planet back up

The CO2 cycle acts as a thermostat that regulates the temperature of the Earth…

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Ocean Circulation• One hypothesis about climate

change is that as the Earth as a whole warms, ocean circulation in the Atlantic will change to produce cooling in Western Europe. In its most extreme form, this hypothesis has advancing European ice sheets triggering a new ice age.

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Clouds• Clouds have an enormous impact on

Earth's climate, reflecting back into space about one third of the total amount of sunlight that hits the Earth's atmosphere. As the atmosphere warms, cloud patterns may change, altering the amount of sunlight absorbed by the Earth. Because clouds are such powerful climate actors, even small changes in average cloud amounts, locations, and type could speed warming, slow it, or even reverse it.

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Aerosols, dust, smoke, and soot.• These come from both human and

natural sources. They also have very different effects on climate. Sulfate aerosols, which result from burning coal, biomass, and volcanic eruptions, tend to cool the Earth. Increasing industrial emissions of sulfates is believed to have caused a cooling trend in the Northern Hemisphere from the 1940s to the 1970s.