A Changing World
Adding it up
A temperature change of just .6 to .8 ° averaged over the globe is enough to lead to significant regional changes to the climate. This section examines observed changes beyond just the global surface temperature increase.
How much energy?
Global warming is a change in the earth’s energy budget, and this can be shown in terms of the various parts of the earth’s surface.
The dark purple bars show the changes just from 1993 to 2003, while the light purple bars show the changes from 1961 to 2003. The overwhelming majority of this additional energy has gone into the oceans. An immense amount of energy is required to raise the temperature of the ocean a fraction of a degree.
Ocean heat content
We can calculate the changing heat content of the ocean. This graph shows the heat content of the first 700 meters over the past 50 years. 
The red and blue lines represent older analyses, while the black line is the most recent analysis, showing a much more consistent rise in ocean heat content.
Ocean acidification and freshening
The oceans are absorbing huge amounts of anthropogenic CO2, shown for three different areas. Although not related to climate, this creates carbonic acid, lowering the pH of the oceans as shown on the right.
The map shows the areas of the ocean where our CO2 has gone, with low amounts shown in purple, medium in green, and high in red.
Another chemical change is the salinity of the ocean due to shifts in precipitation pole ward, with some contribution from melting land ice.
The final chart is somewhat hard to read, but the left scale shows the ocean depth, and the bottom scale shows latitude. The blue strips at 80 °S and 60 °N shows the freshening of ocean water at high latitudes. Other areas have become more salty. The salinity of water alters its density which affects circulation patterns.
As mentioned, precipitation has changed. A greater portion of precipitation tends to occur over a shorter period of time.
The top graph shows the increase in the number of heavy precipitation days (days with precipitation more than 10 mm), while the next graph shows the increasing percentage contribution from very wet days (days in the top 5%).
Regional precipitation has changed, redistributing precipitation to different areas, particularly to higher latitudes. The change in precipitation patterns, combined with increased evaporation has lead to widespread reductions in available water for use in agriculture or as drinking water. This is shown by the Palmer Drought Severity Index (PDSI) below.
Drought has very closely followed increasing temperature.
Permafrost and seasonally frozen ground
The permafrost of the Arctic is melting and seasonally frozen ground has been reduced. This shows the departure from the norm of the active layer of ground above Russian permafrost.
More of the ground is melting every year, so you must go deeper to find the permafrost.
Next shows the reduction in the maximum depth of freeze of seasonally frozen ground.
The ground is not freezing as deep as it used to.
Finally is the frozen ground extent, shown for the Northern Hemisphere.
Less area is freezing year to year as a result of warming.
Often you will hear that “Some glaciers are advancing, and we don’t hear about them,” implying that the public is being kept in the dark about the true status of the world’s glaciers. An example of this is skeptics’ treatment of this study of Himalayan glaciers. The glaciers in the small box in the upper part of this map are advancing, whereas the rest of the glaciers in the Himalayas are retreating.
Whether a glacier is advancing (growing) or retreating (shrinking) largely depends on summer temperature, cloudiness, winter precipitation, and the orientation of the glacier with respect to the Sun and environment. It is certainly possible to get a combination of factors that allow a limited number of glaciers to grow.
The bar graph summarizes the mass balance of 111 glaciers during 2005 and 2006.
Bars to the left of zero show decreasing mass balance, while bars to the right of zero show increasing balance. 92 were retreating, 5 were advancing, and 14 have no data available. Obviously, the retreating glaciers far outnumber the advancing glaciers.
Worldwide, glaciers have been melting since the mid 1800s, or the end of the little ice age. The next graph shows the length of glacial tongues since 1700.
Skeptics will argue that conditions are just returning to normal, but we have shown in section 6, that there has been no significant increase in natural forcings since the mid 1900s.
If you live in the US, Glacier National Park contains the most obvious examples of disappearing glaciers. Glacier Park has already lost all but 27 of the 150 glaciers that existed at the end of the little ice age. Much of the loss occurred in the early 20th Century, when high temperatures and low precipitation caused accelerated melting. However, this melting has continued, and if CO2 concentrations are allowed to grow as predicted, the glaciers will disappear as soon as 2030. This is significant because it is believed that several of the largest glaciers probably survived even the Holocene climate optimum when Northern Hemisphere summer temperatures were warmer than today (see section 5).
We have proxy evidence of the expansion and reduction of glaciers from various parts of the world.
This graph is hard to read but it shows the size of various glaciers during the last 12,000 years. Anything above the horizontal lines shows times when there was less glacial mass than at present. For the Northern Hemisphere glaciers, the reduced mass during the first half of the graph corresponds to the Holocene climate optimum. In the case of Glacier Park, its glaciers are most closely represented by those of the nearby Canadian Cordillera (Canadian Rockies).
What makes the ice loss of today unusual is that all of these glaciers, from all over the world, are retreating at the same time, which has never happened within this period until now.
“Greenland and Antarctica are gaining ice!”
The claims of increased ice extend to Greenland and Antarctica.
There is increased thickening of the Greenland ice sheet, but only in the middle. A warmer atmosphere contains more moisture, and for Greenland this means more snowfall. But the thickening in the interior is more than made up for by the melting along the exterior.
The story is similar in Antarctica.
The melting of west Antarctica, particularly the west Antarctic Peninsula exceeds the slight thickening of east Antarctica. 
In the past, measuring ice balance was more difficult, because the ice flows had to be surveyed individually or regionally. In 2002, the twin gravity measuring GRACE satellites began measuring the actual mass change of the glaciers. A year later, the ICESat satellite was launched which provides satellite laser altimetry and greatly expanded our ability to measure altitude changes. Any notion that the ice sheets are gaining ice has been put to rest.
“Greenland used to be green”
. . . hence the name. To inject a bit of perspective, the Greenland ice sheet is over 3 km thick at its center and is at least 125,000 years old, and probably 400,000 years old.
This is one area of the world where we can say with certainty that there really was a Medieval Warm Period. Eric the Red founded the first Viking colonies in Greenland in 986 AD, after having been banished from Norway some years before. He named it Greenland because, “men will desire much the more to go there if the land has a good name.” Greenland was and is green around the southern edges during the summer, but it is hardly a hospitable place. The Viking colonies were abandoned in the 1400s, probably due to harsher conditions associated with the little ice age.
The remnants of these settlements are still evident today.
Not to make light of the Vikings, but the native Inuit people have lived there for 4500 years.
“But Antarctica is cooling!”
The Arctic has warmed substantially in recent decades, but Antarctica has cooled. Simple calculations imply amplified warming at both poles, but that is based on a generic representation of the world. The Arctic is ocean surrounded by land, while Antarctica is land surrounded by ocean. The Southern Hemisphere warms slower than the Northern Hemisphere, because more of the ocean is in the Southern Hemisphere and as we’ve shown, it takes more energy to warm the ocean.
More sophisticated models in the late ‘80s began predicting cooling as they took into account the deep water mixing of the Southern Ocean. Our current understanding includes the effect of the cooling stratosphere. As we discussed previously, a cooling stratosphere due to ozone destruction and increased CO2 strengthens circumpolar winds and thus ocean currents. For the core of Antarctica, this means further isolation from the rest of the climate system and colder temperatures.
This graphic is the result of satellite data stitched together between 1981 and 2007 and calibrated using the few thermometers on Antarctica.
It only represents the trends in skin temperature, or the temperature of the first few millimeters of the surface, although air and skin temperature are similar. The interior of the continent has cooled, but much of the edges have warmed, especially West Antarctica and the Antarctic Peninsula which are among the fastest warming places on Earth. Not coincidentally, this is where the vast majority of ice mass is being lost. This is also the site of the famous Larsen B ice shelf collapse in 2002.
The edges of East Antarctica have warmed, but not enough to cause the ice loss seen in west Antarctica. The additional moisture has increased snowfall which is the primary reason why the mass balance of East Antarctica has increased slightly, although not nearly enough to offset the melting of West Antarctica.
As ozone begins to recover, the interior of Antarctica is expected to begin warming again, as the buildup of greenhouse gases will outweigh the effect of the colder stratosphere.
How much ice loss?
This graphic summarizes different studies over the years on the mass balance of Greenland.
Vertically, the boxes represent both the magnitude of the mass change and the margin of error. Horizontally, the boxes show the time period of the study. The earliest study shows Greenland essentially in balance, with perhaps a slight thickening. The recent studies indicate accelerating ice loss.
For Antarctica we have a much shorter period of study, but they too indicate substantial ice loss. 
The red box shows the most recent authoritative study on the mass balance of Antarctica.
Sea level rise
Melting snow and ice implies sea level rise, but the largest portion currently is due to the expansion of the ocean due to warmer waters. This chart breaks down the amount of sea level rise based on estimates of different components. 
The blue areas show the rate of sea level rise for 1961 to 2003 while the brown areas show the rate for 1993 to 2003. The width of the boxes represents the margin of error. The bottom part of the chart compares the sum of the components to our observations. The sum and observations overlap, as they should. If anything, observed sea level rise tends to exceed the calculated contributions of the individual components. Sea levels are currently rising over 3 mm per year, which is about 1/8th of an inch.
This graph shows sea level extending back into the 1800s, with uncertainty increasing the further back in time you go back.
The earlier data comes from tide gauges and other observations, while recent data comes from satellite altimetry. The oldest data represent measurements from very few locations. Location matters because the elevation of the land in some areas is changing to due to plate tectonics. In addition, different areas of the ocean are warming at different rates. Another factor is glacial rebound. Areas that were covered in ice during the last ice age are still moving upward in response to the removal of the ice sheets.
Since 1900, sea level has risen about 17 cm, or about 6-7 inches.
Sea ice measurements
In addition to land based ice, there have been changes to sea-ice. Because it is already floating in water, melting sea ice does not affect sea level appreciably, although there are significant consequences for habitat, ocean circulation, and the planet’s albedo.
There are two common ways to measure sea ice. The first is sea ice area. This is simply the total area covered by ice. The other is sea ice extent. If a gridbox is greater than 15% ice, it is considered part of the sea ice extent. For example, if the ice is broken up with gaps of water in between, the entire area, water included, is considered sea ice extent. It is for this reason that sea ice extent is easier to combine with pre-satellite era observational records, because those tend to track the edge of the ice fields.
Before the 1950s we have spotty observational records from the Arctic. From the 1950s to 1971 we have higher quality observational data from numerous organizations. We have good records of the minimum sea ice extent, which occurs in the summer, but poor records of the maximum extent during the winter.
For Antarctica, we have very little to go on before satellites began measuring the Southern Hemisphere sea ice in 1973. There is some proxy evidence that sea ice was significantly reduced from the ‘50s to the early ‘70s but scientists cannot agree if it was widespread or regional.
Since the early ‘70s, however, we have satellite observations. Three different systems have been used, the first from 1972 to 1976, the second beginning in 1978 and the third in 1987.
Northern Hemisphere sea ice extent
The longest satellite record is provided by the NASA Goddard Space Flight Center (GSFC), and is the only analysis that goes back to 1972, with traditional observational data between 1976 and 1978 bridging the gap between the different satellites. This data is limited to sea ice extent, but extent and area move together.
This shows the 12 month moving average of Northern Hemisphere sea ice extent anomalies. Anomalies are the variations from the norm, in this case the average from 1979 to 2000 for each individual month. All of the sea ice graphs created for this section have a vertical scale of covering variation of 30%. The current GSFC analysis ends in 2006 and is updated every couple of years. The data representing 2007 is from the National Snow and Ice Data Center’s (NSIDC) sea ice extent analysis and varies a few percent from the GSFC data depending on the month. By comparing a period of overlap, the NSIDC data was calibrated to match the GSFC analysis.
Lines on a graph are one thing, but reality is a bit different. This shows the sea ice area for September 5th, 1979 versus the same day in 2007.
The dramatic recent ice loss is the result of decades of deteriorating perennial ice, which is the thick, old ice that remains year round. The 2007 Arctic summer was unusually free of clouds. As a result, the ice was under direct sunlight around the clock because there is no night during Arctic summer. In addition, clockwise circulation patterns pushed the ice into warmer waters. The combination of these factors resulted in significant melting of the already weakened ice.
The loss of ice was so great that for the first time in recorded history, the fabled Northwest Passage (circled) opened up for a short time. Explorers first completed the journey in 1906, but it was so treacherous it locked their ship in the ice for three years, and the route used was not practical for commercial use. More recent expeditions used ships with specially reinforced hulls to break through any remaining ice. In this case, any sea going vessel could have sailed right through with no trouble.
In recent years, countries bordering the Arctic have ratcheted up the political rhetoric, with an eye on securing mineral and shipping rights. Russia even went so far as to stage the planting of a Russian flag on the sea floor under the North Pole.
A longer view
For the Arctic, we have observational data reaching back into the late 1800s.
The red dots show the sea ice extent for September until 2005. September is the month when Arctic sea ice reaches its lowest point. The small data point in the lower right hand corner is the approximate value for September 2007. There is no evidence that sea ice has been this low since we began keeping track of it. The blue dots represent the maximum extent in March. The black line and green dotted line show records for specific areas.
Sourthern Hemisphere sea ice extent
The satellite data for Antarctica begins in 1973.
This graph again shows the moving 12 month average of sea ice extent anomalies. It’s notable that SH sea ice is far more variable than the NH ice. It is often said by skeptics that recent years show the largest extent on record, but that is not accurate. They are relying entirely on the more recent datasets of sea ice from 1979 to the present. The largest extent on record occurred in the early ‘70s so there is some evidence to support the proxy evidence showing greater Southern Hemisphere ice until then.
“Why no recent drop in SH sea ice?”
If you ignore the data from the ‘70s, the Southern Hemisphere sea ice has a slight positive trend, shown in the bottom graph. However, during this same time, the surface air temperature of the area surrounding Antarctica has increased, as shown in the top graph.
The recent increase in Southern Hemisphere sea ice is believed to be due to changes in the density of the upper layer of the Southern Ocean. The top layer of the ocean has become warmer and less salty, which reduces its density. A less dense surface layer reduces overturning, which results in less heat transport to the surface. In the case of the Antarctic, the loss of heat from below outweighs the surface warming. The conditions are such that although the growth of ice is reduced, the melting of ice is reduced even further, leading to a net increase in ice volume.
Global sea ice extent
We can add the Southern and Northern Hemispheres together to get a combined overview of the state of polar sea ice. This is the result from 1973 to 2007.
Global sea cover is clearly heading downward. It is often stated by skeptics that the recent uptick in Southern Hemisphere sea ice cancels out the melt from the Northern Hemisphere, but that is not supported by the data over any significant time period.
 (Bindoff, et al., 2007) Figure 5.4. Online here.
 (Bindoff, et al., 2007) Figure 5.9, 5.10, and 5.5 (cropped). Online here
 (Alexander, et al., 2006) (cropped). Online here.
 (Bindoff, et al., 2007) FAQ 3.2, Figure 1. Online here.
 (Lemke, et al., 2007) Figure 4.20 (cropped) and 4.22. Online here.
 (Fowler & Archer, 2006) Online here.
 (wgms-team, 2008) Online here.
 (Hall & Fagre, 2003) Online here.
 (Jansen, et al., 2007) Box 6.3, Figure 1. Online here.
 (Cole, 2006) Online here.
 (Luthcke, 2007) Online here.
 (Sephton, 1880) Online here.
 (Brown, 2000) Online here.
 (Simmon, 2007) Online here.
 (NSIDC, 2002) Online here.
 (Lemke, et al., 2007) Figure 4.18. Online here.
 (Bindoff, et al., 2007) Figure 5.21. Online here.
 Ibid. Figure 5.13
 (Cryosphere Today) Online here.
 (Lemke, et al., 2007) Figure 4.10. Online here.
 (Zhang, 2007) (cropped) Online here. There is also evidence that fluctuations in snowfall influence sea ice, but that is disputed by the Zhang paper. (Powell, Markus, & Stössel, 2005) Online here.
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