Fri., Nov. 4 notes

You could walk by the house late in the afternoon, when the students would likely be outside, and count them. That would be a direct measurement (this would be like measuring temperature with a thermometer). There could still be some errors in your measurement (some students might be inside the house and might not be counted, some of the people outside might not live at the house).

If you were to walk by early in the morning it is likely that the students would be inside sleeping. In that case you might look for other clues (such as the number of empty bottles in the yard) that might give you an idea of how many students lived in that house. You would use these proxy data to come up with an estimate of the number of students inside the house.

In the case of temperature scientists look at a variety of things.

They look at tree rings.

The width of each yearly ring depends on the depends on the temperature and precipitation at the time the ring formed.

Coral is made up of calcium carbonate, a molecule that contains oxygen. The relative amounts of the oxygen-16 and oxygen-18 isotopes depends on the temperature that existed at the time the coral grew.

Scientists can analyze lake bed and ocean sediments.

The types of plant and animal fossils that they find depend on the water temperature at the time. Shells also contain calcium carbonate and can be analyzed to determine the relative amounts of the O-16 and O-18 isotopes.

They can even use the ice cores.

The ice, H₂O, contains oxygen and the relative amounts of oxygen and hydrogen isotopes depends on the temperature at the time the ice fell from the sky as snow.

Here's an idea of how oxygen isotope data can be used to determine past temperature.

During a cold period, the H₂O¹⁶ form of water evaporates more rapidly than the H₂O¹⁸ form. You would find relatively large amounts of O¹⁶ in glacial ice. Since most of the H₂O¹⁸ remains in the ocean, it is found in relatively high amounts in calcium carbonate in ocean sediments. Note also the drop in ocean levels during colder periods when much of the ocean water is found in ice sheets on land.

Next we'll look at some of the predictions for the future. But first here's a summary of where we stand at this point:

There is pretty general agreement that atmospheric CO₂ and other greenhouse gas concentrations are increasing and that the earth is warming.

Not everyone agrees on the causes (natural or manmade) of the warming.

And certainly not everyone agrees on what might happen next.

Predicted Changes in Atmospheric CO₂ Concentrations

Atmospheric carbon dioxide concentrations are currently about 385 ppm (ppm stands for parts per million, 385 ppm is equivalent to 0.0385% concentration). The computer model predictions above show that this could increase to between about 550 and 950 ppm by 2100 (source). The amount of increase will depend on future changes in population and how quickly we can develop new technologies and shift to alternative sources of energy. The A1F1 scenario might be considered a "worst case" scenario in that it assumes continued intensive use of fossil fuels (a jump in CO₂ emissions in 2010 is reportedly worse than even this). The B1 scenario assumes a shift to cleaner and more efficient technologies (the various scenarios used in the predictions are described in more detail here).

The right graph above shows that atmospheric CO₂ concentration keeps increasing for the next century even with fairly significant cuts in CO₂ emission amounts during the next century. To see when atmospheric CO₂ amounts might eventually stabilize at a constant level you need to look further out in time as in the figure below (created by Robert A. Rohde for Global Warming Art).

Keeping CO₂ concentration below 1000 ppm will require that CO₂ emissions peak before the end of the 21st century and that they eventually be cut to less than present day emission rates. The most optimistic scenario (the lower-most curve on the graph) shows that with an immediate cut in CO₂ emissions and a decrease to about 25% of current values would result in concentrations stabilizing at about 450 ppm.

Global Temperature

The model results are the light gold colored lines, the red line shows the mean of the model simulations, and the black line the observed temperature variations (temperature change values on the y-axis are relative to the 1901-1950 mean). Both natural and anthropogenic factors have been included. The four vertical lines indicate major volcanic eruptions, natural processes that cause short duration cooling.

The relatively good agreement between predictions and observations adds some support to the claim that the models are able to realistically simulate the complex physical processes that determine climate.

The figure below shows predicted increases in global average surface temperature relative to the 1980-1999 mean (source). Estimates range from about a 0.6^o C increase (the orange line which assumes that future greenhouse concentrations remain at the 2000 levels, a best case scenario) to about 4^o C (the A1F1 scenario which assumes continued intensive use of fossil fuels, a worst case estimate). These are the same emissions scenarios mentioned earlier (described in detail here ).

The warming isn't expected to be uniform but will occur mainly over land and at higher latitudes (the figure above was prepared for Global Warming Art by Robert A. Rohde).

Melting of Snow and Ice, Sea Level Rise

The images that global warming most often brings to mind perhaps are melting glaciers and polar ice, rising sea level, and flooding of coastal communities.

Pederson Glacier is in the Kenai Fjords National Park, Alaska (this is another of the images created by Robert A. Rohde for Global Warming Art).

Ice (found mostly in Antarctica and Greenland) covers about 10% of the earth's land surface and about 7% of the earth's oceans (much of this is at the N. Pole). Snow covers almost half of North America in the winter.

Melting of glacial ice, snow, and land ice in the figure above will cause sea level to rise. Melting of sea ice (floating ice) will not. This is something you can verify for yourself by putting several ice cubes in a glass then filling up the glass to the brim with water. This is something I'm hoping you might be interested in checking out for yourself.

Put a few ice cubes into a glass and then fill the glass right up to the rim with water. Let the ice melt, the glass won't overflow once that has happened.

Observations do indicate that the amounts of ice and snow have been decreasing, especially since about 1980. During the 1993-2003 time period, melting of ice and snow were increasing sea level by 0.6 to 1.8 millimeters (mm) per year. Past, present, and predicted sea levels are shown in the next figure (source).

We should mention that the predicted rise in sea level comes not only from melting ice but also from thermal expansion of the ocean water.

Several hundred million people live in coastal areas that are at risk from rising sea level (see this gallery of images). Rising sea level can contaminate coastal supplies of fresh water and can harm coastal ecosystems. In addition to causing sea level to rise, a decline in mountain snow and ice could also cause a serious shortages in freshwater supplies for nearby communities and cities.

Changes in Climate and Frequency of Extreme Weather Events
The table below lists some of the changes that may already have occured and/or are expected to occur in the next 100 years or so (source of the information).

Condition or event	Have changes already occurred?	Have human activities contributed to the observed change?	Is the observed change expected to continue during the 21st century?
Fewer cold days & nights over land areas	very likely	likely	virtually certain
More frequent hot days & nights over land areas	very likely	likely (warmer nights)	virtually certain
More frequent warm spells/heat waves	likely	more likely than not	very likely
Frequency of heavy precipitation events increases	likely	more likely than not	very likely
Increase in area affected by droughts.	likely in many areas since 1970	more likely than not	likely

Extreme cold and excessive heat are the two deadliest weather-related causes of death in the US (though there is some uncertainty about which is deadlier). The Chicago Heat Wave of 1995 killed approximately 750 people, the 2003 European Heat Wave killed approximately 40,000 people. It is tempting to use the data above to suggest that the incidence of cold events might decrease while the occurrence of heat spells might increase. This is an example of both good and bad effects coming from climate change. While something like the 2003 event cannot be blamed on climate change, the possibility similar situations might become more common in the future should lead to advance preparations that might minimize the effects they have.

During the past 100 years or so there appears to have been an increase in precipitation amounts observed over land north of 30o N latitude. Globally there has not been a significant increase in precipitation observed.

This figure shows changes in precipitation amounts over the US for the time period 1901to 2005 (source). Somewhat surprisingly, Hawaii is the only location where there has been an overall decrease in precipitation.

There is concern that dry regions might become even drier (warmer temperatures would increase evaporation) and that wet regions could become wetter (increased evaporation will add more moisture to the air, warmer air can hold more moisture when saturated).

Here is an example of model precipitation predictions from the NOAA Geophysical Fluid Dynamics Laboratory (source). This model predicts a global increase in precipitation, an increase in precipitation near the equator and at middle latitudes. The subtropics will experience a decrease in precipitation.

There is some concern that global warming will make hurricanes stronger and more frequent. We will consider that question in the section on hurricanes.

	Spring 2011	Fall 2011
Quiz #1	80%	78%
Quiz #2	75%	74%
Quiz #3	71%	71%
Quiz #4	80%	?