Wednesday, August 26, 2009

Outline for Lecture (Chapter 1) “Here Comes the Sun” Quick overview of syllabus - Friday discussions!!! ( Decreasing sea ice in the northern hemisphere (see p of text) Sea-ice/albedo feedback demo (Scott Kittelman and Joe Caramore) Earth’s albedo (= reflectivity) (p 24, substitute ‘ice’ for white daisies) The Earth as a system – and feedbacks (p 18-20) Graphing (Earth’s temperature since 1870) (Figure 1-4) Correlations of CO 2 and temperature over recent past (p 4,5, 10-13) Graphing the results from the demo (Thanks Carol!)

Northern Hemisphere Sea Ice Minimum (Click on black image to run video)

Demo – Ice Albedo Feedback

…to understand the parts requires that we understand the linkages between them and the processes that are important in determining those linkages. sun air ice/water life land These linkages are really “feedback loops” between the various elements in the system. Today, we are looking at the feedback between the atmosphere and ocean

Earth’s climate has changed measurably since ~1910, and the warmest years in thousands (maybe tens of thousands) of years have occurred in just the past decade! Figure 1-4

Graphing exercise – use your judgment to determine the “best fit” of a straight line to the set of data plotted on your handout. Results from graphing exercise – These data represent the earth’s Global Mean temperature record (i.e., including land and ocean). The time period is The class average was 0.87 degrees (low value 0.5, high value 1.4). The line below is a linear-least squares fit to the same points, yielding a value of 0.79 degrees (with an uncertainty of about 0.2 degrees). Note that the class average is VERY CLOSE to this scientifically more precise estimate of the slope. But is a straight line the best fit?

Actually, a better fit (meaning that more of the points lie closer to the line) is obtained using a third-order polynomial fit to the results. Is this result more defensible? Scientifically, there is no reason why a linear trend would be preferred over a curved one, except that it is interesting that the main contributor to the increase in global mean temperature is considered to be carbon dioxide, a greenhouse gas (we’ll talk about all these terms in this course!)

Can burning of fossil fuels explain these changes?

Indeed, abundances of carbon dioxide (CO 2 ), a ‘greenhouse gas’, have been increasing in the atmosphere for many decades. This is called “the Keeling Mauna Loa Curve” because the measurements were started by Charles Keeling at Mauna Loa Observatory (HI) in At that time, he had no idea what to expect. Figure 1-2

Even longer records, called ‘proxies’, indicate that CO 2 abundances were relatively stable for 1000 years before the industrial revolution. These measurements are from air bubbles trapped in ice cores – a fairly well accepted method Figure 1-3

How does the CO 2 record compare to temperatures? To do this, we need to figure out a way to estimate temperature, because there were no thermometers in 1000 AD! Note that before the invention of the thermometer the uncertainties (the grey ‘fuzz’) were larger, as they should be. Note, the recent rise in temperatures is quite dramatic and rare over the past 1000 years.

Note that temperatures and CO 2 abundances in the atmosphere remained pretty stable over the period AD, then both suddenly rose. We know that CO 2 increased because of fossil fuel burning and deforestation. We will talk about why temperatures are rising – one can’t simply say that because they correlate there is a cause and effect. It takes a lot more work, and scientific testing, to link the rising temperatures to the CO2. We’ll also talk about why the temperatures were decreasing slightly from AD. Clearly, this is mostly a natural phenomenon.

Figure 1-9 Going back even further (450,000 years, in fact), we find that CO 2 and temperatures correlated nearly one-to-one. Ice core records dating back to nearly 500,000 years show a cycle of ‘ice ages’ that were natural – induced by changes in the earth’s orbit around the sun. We also learn that at no time over the past ½ million years were CO 2 amounts in the atmosphere larger than 300 parts in 1 million (I.e. 300 CO 2 molecules for every 1,000,000 molecules of air)

Back to the demo…the ice-albedo feedback

Absorbed Reflected

As ocean warms, sea ice will melt As ice melts, more sunlight is absorbed in the ocean (less is reflected back to space) Absorption of sunlight will further warm water Positive feedback! Can this explain what’s been going on in the Arctic? Likely, but this needs to be studied further – the Earth System is full of feedbacks and surprises, and that’s what makes climate science a fun field to be in!