Global Warming

A Short Discourse Answering Three Questions

Question 1: Is Global Warming Real?

Short Answer: Yes

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."
IPCC 2007: Climate Change 2007: The Physical Science Basis.

"The instrumental temperature record shows that surface warming has taken place across the oceans and land, and that the rate of warming has accelerated over the most recent decades."
Michael E. Mann and Lee R. Kump, 2009. Dire Predictions: Understanding Global Warming. Grove Weidenfeld, New York, 283 p.

"No credible climate scientist now doubts that humans have had an effect on Earth's climate during the last two centuries, primarily by causing increases in the concentrations of greenhouse gases like carbon dioxide and methane in the atmosphere...[T]he so-called global warming debate is not about whether humans are warming climate or whether we will warm climate in future decades - we are warming it."
William F. Ruddiman, 2005. Plows, Plagues & Petroleum: How Humans Took Control of Climate. Princeton University Press, New Jersey, 202 p.

"The world is getting warmer."
John Gribbin, 1990. Hothouse Earth: The Greenhouse Effect and Gaia. Grove Weidenfeld, New York, 283 p.

"Since 1900 a careful study of world temperature records shows that the average temperature has been increasing..."
Gilbert N. Plass, July, 1956. Carbon Dioxide and the Climate. in American Scientist, vol. 44.

The Evidence

Air Temperature: The graph in Figure 1 shows global average temperature for the period of time from 1880 to 2018. While global average temperatures vary from year to year, the overall trend from 1880 to the present is one of increased temperature. That rise in temperature, as shown in the graph, is the strongest evidence for global warming.

Figure 1
1880 to 2018 Average Global Temperature Anomalies

Sea Level: Figure 2 shows the average global mean sea levels from 1880 to 2014. There are slight variations from year to year, but the trend clearly shows an increase in sea level. How can this be explained? There are actually several ways in which sea level can change. Plate tectonic activity, isostatic movement of the crust, glaciation (the advance and retreat of glaciers) and thermal expansion of the oceans can all cause sea level to both increase or decrease. Of these four, only glaciation and thermal expansion can do this globally and over short periods of time (10s to 1000s of years). Warmer temperatures, as shown in the Average Global Temperature graph (Figure 1), are causing glaciers to melt (the melt water then flows into the ocean, causing sea level to rise), and as temperatures warm, ocean water also warms and expands (warming will cause materials to expand, while cooling will cause it to contract). Recent studies have shown that the rise in sea level that has been observed to date is due almost equally to thermal expansion (50.18%), and glacial melt water (49.82%).

Figure 2
1880 to 2014 Average Global Sea Level Rise

Melting Glaciers: Figure 3 shows the amount of glacial ice that has melted each year from 1930 to 2015 (shown by the blue, vertical bars). The scale on the left hand side shows the annual amount of ice that has melted, with values given in units of average meters of water equivalent. The red, solid line corresponds to the right hand scale, and shows a running total or cumulative amount of melted ice. Notice that for most of the years there is a negative mass balance, i.e. melting ice.

What this graph tells us is that for the time period of most glaciers have had a significant reduction in the volume of ice. It has been calculated that over the past 40 years more than 11 trillion tons of ice has been lost from glaciers worldwide. Current measurements have shown that the ice cap on Greenland is losing approximately 100 cubic kilometers of ice every year. That's 24 cubic miles!

What can cause large amounts of ice to melt? Obviously, the easiest, simplest answer is rising temperatures. The warming of global temperatures, as shown in Figure 1, easily accounts for this.

Figure 3
1961 to 2009 Glacier Mass Balance

Ocean Surface Temperature: Figure 4 shows the amount by which ocean surface temperature has risen over the past century. There is a strong correlation between the pattern for ocean temperature and air temperature, as shown in Figure 1. As with melting ice, there are several mechanisms which can cause water to warm, but the one that most directly impacts ocean surface temperatures is the temperature of the atmosphere. Increasing understanding of the global climate has shown that there is a very strong interaction between the atmosphere and the oceans. Both air and ocean are showing the same trend in increasing temperature - again, strong evidence indicating global warming.

Figure 4
1880 to 2018 Ocean Surface Temperature

Other Evidence:
Average Global Land Temperature, 1880 to 2017
Marine Air Temperature, 1880 to 2018
Northern Hemisphere Snow Cover Extent Anomaly: 1966 to Present
Northern Hemisphere Sea Ice Extent for March and September: 1979 to 2017
World Ocean Heat Content: 1957 to 2015
Global Average Surface Humidity Annual Anomalies, 1958 to 2016

Question 2: What is Causing Global Warming?

Short Answer: Global warming is primarily caused by increases in greenhouse gases. This increase is due to human burning of fossil fuels which produces large amounts of carbon dioxide. Additionally, agricultural practices, waste materials (garbage), and industrial processes are producing large amounts of methane.

"[G]lobal greenhouse gases are now under human control...Humans, by rapidly burning fossil fuels, have caused global warming that overwhelms the natural tendency towards the next ice age."
Hansen, James, 2009. Storms of My Grandchildren. Bloomsbury USA, New York, 304 p.

Both carbon dioxide and methane are greenhouse gases. These gases act as insulators in the Earth's atmosphere - they help to retain the heat from the surface that would otherwise be lost to space. How this works is that solar radiation is absorbed by the Earth's surface (land and oceans). This energy is then re-emitted by the ground as longwave infrared radiation. This longwave infrared radiation is absorbed by the greenhouse gases in the atmosphere. Finally, the greenhouse gases re-emitted this absorbed energy as infrared radiation, which then keeps the atmosphere and surface warm. The more carbon dioxide and methane gases there are in the atmosphere, the more longwave infrared radiation that is absorbed and blocked from going out into space. It is like putting additional blankets on your bed in the winter. A thin blanket keeps you warmer than just a sheet by blocking more of the heat that your body is emitting. More blankets (more carbon dioxide and methane) keep you even warmer by blocking even more of that emitted energy.

Since the beginning of the Industrial Revolution, humans have been burning increasing amounts of coal, oil, and natural gas. This produces increasing amounts of carbon dioxide which are being released into the atmosphere.

Both carbon dioxide and methane are also produced naturally and go through natural cycles of increase and decrease. These natural cycles and natural processes, however, cannot account for the rapid rise over the past century, nor the current levels of these gases that are found in the atmosphere today. Only human activities can account for such extreme, high levels of these gases.

The Vostok Carbon Dioxide graph (Figure 5) shows the natural cycle of carbon dioxide over the past 400,000 years up until the very far right of the gragh. This data is collected from air samples that become trapped in the glaciers on Antarctica. As snow accumulates, the snowflakes become compacted. Little pockets of air become trapped in the ice. By drilling into the ice, samples can be collected and analyzed. These are actual samples of the Earth's atmosphere from the past. Notice on this graph, that the highest levels to which carbon dioxide reaches is just under 300 ppm (parts per million - this means for every one million molecules of nitrogen, oxygen, water, argon, and many other gases normally found in the Earth's atmosphere, 300 of them are carbon dioxide).

The far right side of the graph shows the most current measurements of carbon dioxide in the atmosphere. Modern measurements began in 1958 with a level of 315 ppm. Notice that this value is already well above the highest levels seen in the past 400,000 years (see Figure 5). Today's measurements is 392 ppm. There is a clear trend of increasing amounts of carbon dioxide. Such high levels today cannot be accounted for by natural processes. The burning of coal, oil, oil products such as gasoline, and natural gas can account for much of this increase. Deforestation can also accounts for some of this rise.

Figure 5
CO2 Levels from Vostok Ice Core Data

The same pattern holds true for methane in the atmosphere. The Vostok Methane graph (Figure 6) shows the natural cycle of methane over the past 400,000 years. This data is also collected from air samples that become trapped in the glaciers on Antarctica. Notice on this graph, that the highest levels to which methane reaches is just under 800 ppb (parts per billion - this means for every one billion molecules in the atmosphere just under 800 of them are methane). There is a lot less methane in the atmosphere than carbon dioxide, but it is still a very important greenhouse gas, as it is approximately 30 times more efficient at absorbing and blocking infrared radiation than carbon dioxide.

Figure 6

The far right side of the graph (Figure 6) shows the current measurements of methane in the atmosphere from Alaska and Hawaii. Notice that the levels are more than double the levels seen during the past 400,000 years, reaching 1850 ppb! Again, this extremely high level today cannot be explained by natural processes. Human activities, however, can. Large numbers of livestock produce methane through digestion processes (as do nearly 7 billion humans on the planet!). Large amounts of waste and garbage produce methane due to decomposition, as well as many industrial practices. Manure decomposition from livestock feed lots, agricultural practices, and industrial processes can also account for some of this methane. Methane forms a major component of what we call "natural gas". Currently, we burn a large amount of methane for heating and cooking, as well as generating electricity. While burning natural gas produces less carbon dioxide than burning coal, it still emits carbon dioxide into the atmosphere.

For both carbon dioxide and methane, the natural cycles for these gases cannot account for the extremely high levels that are measured in the atmosphere today. Human activity can account for these high levels. Therefore, humans, through our agricultural activities over the past several millenium and industrial activities over the past two centuries, are the cause of greenhouse gases increasing in the atmosphere, which has resulted in the current trend in global warming.

Question 3: What will happen in the near future?

Short Answer: The world will continue to get warmer. Sea level will continue to rise. Glaciers will continue to melt.

"I've come to conclude that if we burn all reserves of oil, gas and coal, there is a substantial chance we will initiate the runaway greenhouse."
"[T]he strength of the strongest storms will increase as global warming increases."
"[W]hen large sea level change will begin is during the lifetime of my grandchildren - or perhaps your children."
Hansen, James, 2009. Storms of My Grandchildren. Bloomsbury USA, New York, 304 p.

The Earth is a very complex system. This makes predicting the specific future conditions very difficult. However, based on computer models and an analysis of past climate changes, some general trends can be predicted with some measure of reliability. As long as we continue to burn fossil fuels such as coal and oil, carbon dioxide levels will continue to rise. More carbon dioxide means more energy is retained and the Earth will continue to warm. The best models today indicate that by the year 2100, average global temperature could be anywhere from 2° to 6° degrees Celsius (3.6° to 10.8° Fahrenheit) higher than today. As temperatures continue to rise, more of the ice in glaciers will melt. Already today, many small alpine glaciers are rapidly disappearing. Many of these will not last until the end of the decade. As the atmosphere and ocean water becomes warmer, the two largest glaciers on Greenland and Antarctica could become unstable and begin to rapidly collapse. As the ice melts and as the oceans become warmer, sea level will continue to rise. How much sea level will rise by the end of the century is a topic of much debate. Conservative estimates put the rise from several 10s of centimeters to upwards of one meter (39 inches) or more. If glaciers melt rapidly, sea level may rise several meters. If all of the glaciers melt, sea level would rise 75 meters (250 feet)!

The consequences of increased global temperature, melting glaciers, and rising sea level to human life, culture, economics and the environment will be widespread and potentially very dire (see some of the recommended reading for further discussions on these consequences).

Figure 7

Figure 8

Further Reading

Hansen, James, 2009. Storms of My Grandchildren. Bloomsbury USA, New York, 304 p.
     For more information: Storms of My Grandchildren
     For more information: Updating the Climate Science

IPCC 2007: Climate Change 2007: The Physical Science Basis.

Michael E. Mann and Lee R. Kump, 2009. Dire Predictions: Understanding Global Warming. Grove Weidenfeld, New York, 283 p.

Stephen H. Schneider, 2009. Science as a Contact Sport. National Geographic Society, Washington D.C., 295 p.

Peter D. Ward, 2010. The Flooded Earth: Our Future in a World Without Ice Caps. Basic Books, New York, 272 p.

William F. Ruddiman, 2005. Plows, Plagues & Petroleum: How Humans Took Control of Climate. Princeton University Press, New Jersey, 202 p.

National Science Academy and the Royal Society, 2014. Climate Change: Evidence and Causes. National Academies Press, Washington D.C., 36 p.