Sea Levels

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4.2.2.a

Sea Levels – Back to the future!

We begin with modern and past observations and then move to predictions.
“Hindcast and then forecast” as the meteorologists say!
To understand the past effects of sea levels we must examine modern research.

Present-day studies help us to interpret the planet’s history found in the rocks. Studying the Earth’s history of continent formations and sea levels through geological studies shows us the effects of higher sea levels.
It also prepares us for the higher sea levels projected for the climate change that is now underway. The first climate change that has been caused by humans.

Changing sea levels under uneven global warming conditions is largely due to the melting of ice around the globe. The water cycle will continue to function but water will spend time in different forms, as liquid and vapor as the ice melts.

The graph below tracks the global differences in temperature from 1860 to 2000.



Source: IPCC 2001. Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. Fig 2.8 P. 115

Scientists suggest the use of an error margin for the past century of +/-0.2° C. The areas in gray reflect a two standard deviations error range. Different shades of color indicate different averaging techniques.
Results show a pronounced warming between 1910 and 1945, a modest cooling between 1945 and 1975, and a return to rapid warming ever since 1975.

ACTIVITY 1
1. When was the beginning of the industrial revolution and burning of fossil fuels?
2. What major changes or events in the world were happening in these time periods?
Hint: Check both geography and history for these periods.
a. 1910 to 1945?
b. 1945 to 1975?
c.1975 to 2000?
3.Is there a correlation between human activity, world events and volcanic activity ?
4.Compare the angles of the a) and c) increases. Which has the greater rate of increase in warming?

The changes in temperature 1950-1998 are distributed unevenly. The pattern of temperature change over Canada and elsewhere is complex. Here is a view from the poles.

Source: Graph prepared by Climate Research Branch (MSC) staff using University of East Anglia data.

During the past 50 years, changes in atmospheric circulation, and hence in wind directions, have caused some regions to change temperature. Some have warmed much more than the hemispheric average of 0.3° C. Some others have cooled significantly. Notice the red areas near the North Pole.

ACTIVITY 2
1.Find Ontario on the above map. What is the range of the color legend that is shown on or near the provincial boundaries?
2.What does this mean as a 50-year trend for Ontario?
3.What is the big picture for Canada?

Changes in the depth of Arctic sea ice over 2 time periods.

Source: Rothrock et al GRL 26:3471 (1999)

Submarine surveys use ‘ice draft’ as proxies for Arctic ice thickness. This data does not include information over shallow regions of the Arctic Ocean’s continental shelves.

All areas monitored show more thinning between 1993 and 1997. The average decrease is about 40%. The greatest changes occur in the eastern Arctic Ocean.

These regional changes may be at least partly related to a long-term natural cycle called the Arctic Oscillation (AO). This means that future decades could see a reversal of this pattern.

The area of the sea ice in the northern hemispheres is shown as seasonal and annual records below.


Source: Compilation produced by John Walsh, similar to Figure 2.15 in IPCC 2001. Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. p 125.

The black curve is the annual mean for the area of Northern Hemphere ice cover for the twentieth century. This data is based mostly on operational ice analysis. Satellite data has been added recently.

ACTIVITY 3
1. In which season(s) did the greatest decrease occur?
2. What year did the decline begin?

Shrinking ice area over time!

Source: Vinnikov et al 1999, Science 286:1934; IPCC 2001, WGI p125;

Comiso, 2002, GRL 29 (20): 17-1 to 17-4.

This graph of satellite observations shows changes in average annual area covered by sea ice. Check 1970 on the graph above this one. Late summer sea ice cover over the Arctic region has decreased by more than 15% since 1950.

These changes are almost impossible to explain on the basis of natural variability alone.

A similar decrease in the area covered by sea ice is has not occurred during the last century.

Check this with the graph below showing first the observations of the area covered by the Arctic ice and then the predictions for the area of Arctic ice cover for 2100.

This graph is derived from climate models using historical changes in greenhouse gas and sulphate aerosol concentrations. This decline in the area of Arctic ice shows that the loss is probably due climate change and not natural variation.


Source: MSC Climate Research Branch

Further research predicts this may be as soon as 2050. Note the green dots on the right diagram for 2060.


Source: MSC Climate Research Branch

In late winter, ice cover is expected to shrink significantly. In late summer, by 2050, almost all ice will have disappeared. This suggests no multi-year ice and much thinner ice conditions in the Canadian Arctic.

Sea levels will rise due to ocean thermal expansion and the change in land ice volume.

Source: IPCC 2001. Climate Change 2001: Synthesis Report. Contribution of Working Groups I, II and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Watson,R.T et al. (eds.)]. Cambridge University Press, Cambridge, UK. p 74.

The bands of dark and light grey in the above figure reflect the role of human emission scenarios and climate model uncertainties in future rises in global average sea levels.

The added range within the unshaded area is due to uncertainties with respect to land ice change and ocean thermal expansion in response to a given change in global temperatures.

There are three main reasons for the rise in sea level:
1. thermal expansion of sea water as it warms;
2. melting of temperate glaciers;
3. net mass balance loss of polar ice sheets.

Sea levels are expected to continue to rise for centuries after climate is stabilized. This is because of the long delay of penetration of heat into the deep oceans and the slow response of ice to warmer climates.

Here are the projections for the different oceans over the next century as they respond differently to climate change.


Source: Adapted from Stocker and Schmittner 1997. Nature 397:862-5.

The varied rates of expansion are due to different rates of regional ocean warming and to local changes in atmospheric pressure patterns.

These regional differences will also be affected in specific areas along the Great Ocean conveyor belt.

The Great Ocean conveyor belt runs very deep but reacts with the surface in some locations. Check the diagram below.

Source: www.ipcc.ch

ACTIVITY 4
1. Name the two places where heat is released to the atmosphere.
2. Where does the ocean conveyor belt absorb heat? Hint: check changes in color.
3. How is polar ice involved in this conveyor belt?

ACTIVITY 5 Research
1. Why are these sites involved in thermohaline circulation (THC)?
2. What phenomena occur annually in the mid-Pacific shown here?

The North Atlantic Gulf Stream is caused by climate conditions that drive the global ocean circulation system like a huge conveyor belt. This conveyor belt brings heat to the North Atlantic. This helps to keep western Europe warm.

Source: Adapted from figure prepared by Beauchemin (OURANOS)

Models predict that the Gulf Stream will gradually slow down as the climate warms.

There is a risk, particularly if climate change is rapid or large, that the Gulf Stream could abruptly stop and remain shut down for centuries.

Such a dramatic shut-down of the thermohaline circulation (THC) could have major consequences for future regional climates, including that of eastern Canada. These are not well understood. Check the past to see previous changes of global ocean currents.

Studies with simple ocean models indicate that the risk of THC shutdown increases with the rate and size of the increase in greenhouse gas concentrations.

If a THC shutdown occurs, recovery will not take place for many centuries.

Wave heights in the North Atlantic have also been studied. Winter North Atlantic wave heights have increased in the northeast and decreased at mid latitudes. The map below shows the mathematically significant wave height trend pattern 1958-1997.

Source: Climate Research Branch CRB – (MSC)

ACTIVITY 6
1. Using the Ocean Conveyor belt diagram, locate the spot on this map where the THC and Gulf Stream are at work.

This analysis is based on a 42-year wind and wave history or hindcast of the North Atlantic carried out by CRB. This is considered to be the most sophisticated, accurate, and extensive wave analysis in the world.

Trend patterns for the winter (JFM) show a 90-percentile significant wave height in the North Atlantic. The patterns show an increase in the northeast between 1958 and 1997 of about 2-6 cm/year and a decrease of 1-3 cm/year in the subtropical region.

The hindcast is the focus of collaborative studies with Oxford University on trends and variability in design wave estimates. The US Army Corps of Engineers study eastern North American coastal zone processes.

The studies also provide information to the offshore oil and gas industry for all areas of the North Atlantic (notably Canadian east coast, Faroes, Shetlands, Norwegian Sea, Morocco) and to Canadian regulatory agencies. All of these interested groups must look at the future behavior of the ocean under climate change.

This study is also a benchmark for the Comprehensive Atmosphere-Ocean Data Set (COADS) marine climatology program of NOAA in a joint project.

The risk of shutdown of thermohaline ciculation (YHC) in the next 1000 years is graphed below. The Great Ocean conveyor belt normally turns over the world’s oceans each millennium.

Source: Adapted from Stocker and Schmittner 1997. Nature 397:862-5.

The West Antarctic Ice Sheet contains enough ice to raise global sea levels an additional 6 meters, and is considered unstable. There is an estimated 2% chance that this ice sheet will collapse during the next century due to climate change.

Source: Ice sheet photo (Ellesmere) – H. Hengeveld, Environment Canada

Temperature rises during the next century may trigger an irreversible process causing complete collapse of the Antarctic Ice Sheet over subsequent centuries. Prince Edward Island would then become the Prince Edward Islands. Some other islands on the globe will disappear.

Some areas are getting too warm already! Changing sea levels also affect coral reefs.

Source: Boiseau and Ghil 1999, GRL 28:2883

Oxygen isotopes (O18) were used to study change in coral reefs in the south-central Pacific (French Polynesia). Analysis suggests a gradual warming of ocean waters, particularly during the last 60 years.

Economically speaking!

There is already considerable evidence that impacts of climate change are having some serious economic consequences.

As the climate becomes unstable due to increased temperatures, the number, size, and frequency of extreme events also increase. The global cost of natural disasters is also rising.

The re-insurance industry has provided evidence of a 10-fold increase in global losses due to natural disasters, after allowing for inflation. Most of the disasters are weather related.

Experts note that much of this increase is related to evidence of more frequent and intense climate extremes.

The graph below tracks global losses from 1950-1999.

Source: IPCC, 2001. Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II and II to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Watson, R.T. and the Core Writing Teams (eds.)]. Cambridge University Press, Cambridge, UK. p 422, based on data from Munich Re and IDNDR.

Economic losses due to natural disasters in North America, primarily weather-related, have also increased 10-fold (in constant dollars) over recent decades.

Source: IPCC, 2001. Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II and II to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Watson, R.T. and the Core Writing Teams (eds.)]. Cambridge University Press, Cambridge, UK. p 77

Demographic changes, including increased exposure of capital to disasters, are a major factor in increasing costs. Insurance industry experts believe that much of the increase is linked to the increased frequency and intensity of extreme natural events.

Future projections of many climate extremes remain uncertain. Most studies suggest that rapid climate change will greatly increase the risks of climate extremes and related economic losses.

The last 1000 years are graphed below. The wide gray band indicates uncertainty in the paleoproxy data ( not measured directly by instruments in red), but rather indirectly by ice cores, pollen cores, insect jaw bones in lakes, ancient tree rings, oral history passed from early people, etc.

Source: IPCC 2001. Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. p 134.

Several studies have used a variety of proxy indicators, based on ice core, tree ring and coral data, to reconstruct Northern Hemisphere surface air and sea surface temperatures.

The shaded area indicates the range of uncertainty in the proxy data reconstructions.

The studies show agreement on the presence of a Little Ice Age, which peaked between 1600 and 1900. There is little evidence for a hemispheric-scale Medieval Warm Period.

All studies indicate that the twentieth century was likely the warmest century, and the 1990s the warmest decade, of the past millennium.

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