In the first part of this lab, you learned the following: (1) the difference between climate variability and climate change; (2) the type of recent climate change we have experienced over the past several decades has been global warming; (3) the most warming has occurred in the high latitudes of the Northern Hemisphere; (4) the major cause of the warming has been an increase in greenhouse-gas concentrations; (5) much more excessive energy has gone into the oceans than the atmosphere (i.e., global warming); (6) the ocean has therefore also has warmed; and (7) increased evaporation from the oceans and increased air temperatures has contributed to increased levels specific humidity. The figures below show from left to right the energy accumulation in the oceans, the warming of the oceans, the warming global atmosphere, increased specific humidity, and the higher rates of lower-troposphere warming at the North Hemisphere high latitudes compared to the rest of the globe.
By the end of this lab, you should be able to answer the following research questions:
How are recent changes to the cryosphere and oceans similar to natural changes to the cryosphere and oceans when going from a glacial period to an interglacial period?
How are changes to cryosphere evidence for global warming over the past several decades?
What are the causes of the rise in global sea level over recent decades and how will continued warming of the Arctic region impact global sea level?
Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:
Fact based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor, (2) found in background sections, or (3) determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.
Synthesis based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.
Hypothesis based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.
Section 2: Overview of the Cryosphere
In previous labs, you observed that a warming of the planet causes a disappearance of the cryosphere (i.e., the frozen parts of Earth). The components of the cryosphere are shown in the graphic below, and — based on your experience with Labs 5 and 6 — you should be most familiar with ice sheets and glaciers in the context of climate. The other major cryospheric components include ice shelves, sea ice, snow, and frozen ground.
The cryosphere was its maximum size during the Last Glacial Maximum (LGM), which occurred approximately 21,000 years ago, and you have explored the factors that caused the LGM and the transition to an interglacial period in Lab 6. The Google Earth link below shows the extent of ice sheets, glaciers, and ice shelves during the LGM. The mean sea level rose 120 meters from the LGM to the beginning of the last millennium; this was an average sea-level rise of 6 mm per year.
The current cryosphere is much smaller than the cryosphere that existed during the LGM, but we are still in an ice age (i.e., the Antarctic and Greenland ice sheets still exist). The table below shows the expanses of the components of the cryosphere and their sea level equivalent, which is how much the sea level would rise with the melting of the ice on land.
Q1: In what ice sheet is nearly 90% of ice on Earth?
Q2: If all the ice on land were to instantly melt, how much would the global sea level rise?
Section 3: Sea Ice
Sea ice is frozen seawater and it reaches a maximum (minimum) extent in March (September) in the Northern Hemisphere and a maximum (minimum) extent in September (March) in the Southern Hemisphere. Since sea ice floats on water, the melting of sea ice has no effect on sea level. The video below shows changes in minimum sea-ice extent from 1979 to 2013 in the Northern Hemisphere.
Q3: How and why has Arctic summer sea ice changed over the past several decades?
Open Sea Ice in Google Earth and notice that recent warming is not affecting maximum sea-ice extent in the Northern Hemisphere.
Q4: What country’s long Arctic coastline has become most ice-free during late summer?
Q5: Why hasn’t the March sea-ice extent decreased over the past several decades?
Section 4: Snow Cover
Snow cover in the Northern Hemisphere is a seasonal phenomenon: it has its greatest extent in the winter and the smallest extent in the summer. Watch the animation below to see monthly change in snow cover from 2000 to 2014.
Q6: During what season in the Northern Hemisphere does snow cover begin to disappear from lower latitudes?
Q7: How and why has spring snow-cover extent changed over recent decades?
Section 5: Permafrost
Another component of the cryosphere is permafrost (i.e., permanently frozen ground), which is soil, sediment, or rock that remains at or below 0° C for at least two years. Permafrost is mostly confined to the Northern Hemisphere, where it covers approximately 25% of the hemisphere’s land surface. That means 22.79 million km-2 in the Northern Hemisphere is permafrost. The figures below show the change in permaforst temperature, the locations of permafrost, and Earth climate types.
Q8: Under what climate types do you typically find continuous permafrost?
Q9: How and why has permafrost changed over the past several decades?
The loss of permafrost is one of the primary changes occurring in the tundra biome. The figure and table below show observed geographical shifts in tundra and other biomes during the 20th century.
Q10: What has been the most common biome shift resulting from a changing climate?
Q11: Where (e.g., latitudinal zone, climate type, etc.) and why have these shifts occurred?
Just as the melting of snow and ice on Earth’s surface leads to the ice-albedo feedback, the melting of permafrost also leads to positive feedback loops as discussed in the video below.
Q12: How can the melting of permafrost affect the atmosphere and the greenhouse effect?
Section 6: Glaciers
Fallen snow that compresses over the course of many years produces glaciers. Therefore, glaciers are small versions of ice sheets. There are roughly 170,000 glaciers scattered across Earth (see below).
Q13: In what hemisphere are most of the glaciers located?
Q14: Based on what you know about regional variations in atmospheric warming, which hemisphere should have had the most glacier melting over the past several decades?
Q15: If all the glaciers on Earth were to melt, by how much would the global mean sea level increase?
Changes in glacier length and mass over time are shown in the images below. Keep in mind that the loss of one gigaton (Gt) of ice, which is equivalent to a 1 km3 cube of liquid water, would raise the global mean sea level by just 2.78 µm. The ocean has a huge volume of water.
Q17: Approximately how much glacier ice was lost from 1961 to 2010 and by how much did global mean sea level rise as a result of this?
Let’s focus on the Muir Glacier, one of the roughly 170,000 glaciers worldwide. This glacier is in southeastern Alaska and it used to be labeled a tidewater glacier (i.e., a glacier that terminates in the ocean), but it has retreated so much that it is now classified as a valley glacier (i.e., a mountainous glacier) whose flow is confined by valley walls). Click on thumbnail on the left below to view the Muir Glacier in Google Earth and click on the thumbnail on the right below to see how Muir Glacier changed from 1941 to 2014.
Section 7: Ice Sheets
An ice sheet is a mass of glacial land ice extending more than 50,000 square kilometers, and the two ice sheets on Earth today cover most of Greenland and Antarctica. While both ice sheets are huge and have portions that are more than 3 km thick, the Antarctic ice sheet has approximately eight times as much ice as the Greenland ice sheet. In Lab 6, you encountered the Vostok ice core, which is under the Antarctic ice sheet, and you discovered that during the LGM ice sheets in the Northern Hemisphere also covered much of North America and Scandinavia.
Q19: Which ice sheet do you think lost more ice due to recent global warming? Why?
Since 2003, researchers have been able to accurately estimate changes in the thicknesses of ice sheets by using data from the GRACE (Gravity Recovery and Climate Experiment) satellites. The figure below shows changes in the Greenland and Antarctic sheets from 2003 to 2012.
Q20: How did the Greenland ice sheet change from 2003-2012?
Q21: How did the East Antarctic ice sheet change from 2003-2012?
Q22: How did the West Antarctic ice sheet change from 2003-2012?
Q23: What other factors – besides atmospheric warming – could have caused the West Antarctic ice sheet to change the way it did? Feel free to search the internet.
Both ice sheets undergo melting in the summer, and the video below shows the massive amounts of melting that occur on the Greenland ice sheet.
Q24: How do the glacier lakes promote the further melting and loss of Greenland ice?
Section 8: Sea Level Rise
The two main causes of sea-level rise attributed to global warming are (1) the thermal expansion of ocean water; and (2) input of fresh water from the melting cryosphere. Sea-level rise due the thermal expansion of water is of the oceans is known as thermosteric component. Sea-level data prior to the 1990s relied almost entirely on measurements made at tide gauges, which have changed greatly in numbers over the years and are located mostly along coastlines. Beginning in August 1992 with the launch of the TOPEX/Poseidon satellite, altimetric measurements of practically the entire ice-free ocean were available; this satellite stopped making measurements in 2006. The Jason-I satellite began making measurements in 2001.
Q25: How and why has sea level changed over recent decades?
Q26: What was the major cause of sea-level change in the 1980s?
The table below shows that from 1970-2010 the global mean sea level rose 2 mm yr-1, while from 1993-2010 it rose at a faster rate of 3.2 mm yr-1. Besides showing the rise in sea level from thermal expansion and the melting of the cryosphere, the table also shows the contribution to sea-level rise from land-water storage (i.e., groundwater depletion and reservoir storage).
Q27: What was the largest contributor to sea level rise from 1971-2010 and from 1993-2010?
Q28: What was a larger contribution to sea level rise from 1993 to 2010, thermal expansion or ice melting? How do you know this?
Section 9: Summary
What you just explored were changes to the cryosphere caused primarily by an imbalance in Earth’s energy budget that has existed for several decades. This imbalance has led to increased energy going into the oceans, atmosphere, and land. A summary of changes to the cryosphere is shown below.
Q29: What three aspects of the cryosphere — as shown in the figure above — were not examined in this lab?
Changes to the cryosphere can be combined with changes to the atmosphere and hydrosphere to obtain a comprehensive picture (see below) of changes to the climate systems caused by the imbalance in Earth’s energy budget.
Q30: What has increased due to the energy imbalance?
Q31: What has decreased due to the energy imbalance?
Before the next lab, write for yourself a one-sentence response to each of the following big questions of this lab.
How are changes to cryosphere evidence for global warming over the past several decades?
What are the causes of the rise in global sea level over recent decades and how will continued warming of the Arctic region affect global sea level?