Earth's Temperature: a Brief History of Recent Changes
1. Earth's Temperature Changes During the Last 420,000 Years
Recap of Last 420,000 Years:
Approximate temperature range:
Low: 5°C ... 41°F
High: 17°C ... 63°F
Range: 12°C ... 22°F
Atmospheric Carbine Dioxide less than 300 ppmv
In the Iast 10,000 years:
Hunter-gathers became farmers
Plant and animal species were domesticated
Civilizations were developed
Global mean temperatures probably did not vary by more than 1°C ... 2°F in any 100-year period
The surface temperature of the Earth tends to go up rapidly and then settle down again in cycles of roughly 100,000 years as shown above in this UN Environment Programme (UNEP) chart. The blue line traces the temperature differences (in degrees Celsius) over the last 420,000 years compared to the present time, defined as the year 1950.
In 1950, according to NASA's Goddard Institute for Space Studies, the mean surface temperature of the Earth was 14° Celsius or 57° Fahrenheit. Thus the Earth's absolute temperature (as opposed to its temperature change) during the last 420,000 years varied from a low of about 5°C or 41°F to a high of about 17°C or 63°F, a range of approximately 12°C or 22°F.
Although this range is no more than what most of us experience in the course of a year's time going from summer to winter, UNEP refers to these temperature changes as "very significant" and to the Earth's climate during most of these years as "unstable." Towards the bottom of the range the temperature was cold enough for glaciers to increase in size and at the top was warm enough for glaciers to be reduced by melting. The colder years are referred to as periods of glaciation and the warmer years as periods of interglaciation.
Although estimates of the age of our species, homo sapiens, vary widely, 420,000 years probably covers most if not all of our existence on this planet. But it was only 10,000 or so years ago that we learned how to grow our own food, a development that led to the creation of fixed communities, the division of labor and all the benefits of what we call civilization.
This last 10,000 years (see vertical red line drawn on the population chart above), called the Holocene Epoch, has been one of interglaciation where temperatures, compared to the prior 410,000 years, have been remarkably stable. Throughout the Holocene, according to UNEP, "based on the incomplete evidence available, it is unlikely that global mean temperatures have varied by more than 1°C [2°F] in a century."
If we were to trace the concentration of carbon dioxide (C02) in the atmosphere over the same 420,000 years, we would find a very similar pattern to surface temperature. Atmospheric carbon dioxide is the most voluminous of the so-called greenhouse gases that absorb heat radiated from the ground and then radiate some of it back to the Earth's surface, keeping the Earth warmer than it otherwise would be. Without greenhouse gases and atmospheric water vapor (which serves the same function), the mean temperature of the Earth would be about 0°F or minus 18°C instead of (in 1950) 57°F or 14°C.
The concentration of carbon dioxide in the atmosphere during the last 420,000 was never as high as 300 parts per million by volume (ppmv) until about a century ago when it rose about 300 ppmv, reaching 311 ppmv in 1950. It has been rising ever since.
The 100,000 year cycles of Earth's temperature shown in the UNEP charts was first hypothesized by a Serbian astrophysicist and mathematician named Milutin Milankovitch in the 1920s and ‘30s.
2. Milankovitch Theory of Earth's Temperature Changes
Building on the work of other scientists who had observed that the Earth's orbit around the Sun was irregular in three particular aspects, Milankovitch created a model to show how the amount of sunlight (solar radiation) reaching the Earth varied according to the interaction of the cycles of these three irregularities, which are described below:
1. Eccentricity: In a cycle of roughly 100,000 years, the shape of the Earth's orbit around the Sun varies from an almost perfect circle to a slightly more elliptical shape, with the Sun nearer one end (rather than in the middle), and then back to a more circular shape.
2. Obliquity: In a cycle of about 40,000 years, the tilt of the Earth's axis relative to the plane of its orbit around the Sun varies from 22.1 degrees to 24.5 degrees and back.
3. Precession: In a cycle of a little over 20,000 years, the point of the Earth's axis wobbles so that the north axis points now to Polaris (the North Star) but will eventually point to Vega before returning to Polaris.
But Milankovitch wasn't just interested in tracking changes in sunlight with his model -- he wanted to explain why ice ages occurred, why at various times in the history of the Earth glaciers were formed and later melted away.
The eccentricity cycle affects how much more sunlight the Earth receives when it is closest to the Sun (perihelion) than when it is furthest from the Sun (aphelion) and also enhances or decreases the effect on sunlight of the other two irregularities. But it is not strong enough to create our seasons.
The obliquity cycle creates our seasons. The greater the tilt, the more pronounced the seasons -- warmer summers, colder winters.
The precession cycle causes the seasons to migrate, which is why it is also called the Precession of the Equinoxes -- and why the Age of Pisces will eventually pass the torch to the Age of Aquarius.
To that end he constructed a mathematical model for the 600,000 years prior to 1800 that calculated solar radiation and surface temperatures at particular latitudes, particularly Latitude 65° North -- the latitude of Fairbanks, Alaska and Oulu, Finland -- in the month of July. His theory was that in cooler summers the winter snows did not completely melt but over time accumulated and led to glaciation.
As might occur, say, when the Earth's orbit is maximally elliptical, obliquity is minimal (less tilt, cooler summers) and the Northern Hemisphere's summer occurs when the Earth is furthest from the sun.
Milankovitch's theory was largely ignored until, in 1976, a study based on deep-sea sediment cores in Antarctica substantiated that changes in temperature going back 450,000 years largely conformed to changes in the Earth's orbit. The eccentricity, obliquity and precession variations in Milankovitch's model are said to have accounted for, respectively, 50%, 25% and 10% of temperature change. Milankovitch's theory is now accepted as the best explanation of climate change "on time scales of tens of thousands of years." And the theory suggests that it is about time for Earth to begin a new long-term cooling cycle.
3. Current Increase in Earth's Temperature
In 1967 a Russian scientist named Mikhail Budyko made a prediction: increasing man-made carbon dioxide in the atmosphere would overcome any cooling effects in the near future and cause Earth's temperature to increase.
By coincidence, that same year a young Iowan named James Hansen joined NASA's Goddard Institute for Space Studies in New York City as a research associate. He had just completed his doctoral thesis on the atmosphere of the planet Venus where carbon dioxide was dense and the surface temperature was a scorching 460°C (860°F), Now he was assigned to the question raised by Budyko -- could climate forcings (as they're called) from human causes cancel out natural forcings of cooler temperatures and cause global warming in the near future?
Hansen and his colleagues built a simple climate model reflecting various assumptions of human activity. What they found was, in Hansen's words, "that human-produced greenhouse gases should become a dominant forcing and even exceed other climate forcings, such as volcanoes or the Sun, at some point in the future." When? They didn't know.
They started collecting temperature data from weather stations around the world. Finally, in 1981, in an analysis published in Science and referenced in a front page article in the New York Times, they confirmed Budyko's prediction, showing that temperatures had started rising a decade before.
In 1988, on a record-breaking hot summer day in Washington, D.C., having "weighed the costs of being wrong versus the costs of not talking," Hansen testified before Congress that he was 99% confident we were in a long-term warming trend and that he suspected greenhouse gases were causing it. His testimony and statements to reporters afterward were widely reported in the media. Global warming had gone public.
In the two decades since Hansen's testimony, the increases in both greenhouses gases and temperature have accelerated. The chart below from a recent NASA report compares the situation today to 1880, a century or so after the (human-made) Industrial Revolution got underway. Atmospheric carbine dioxide, the major culprit among greenhouse gases, reached 384 ppmv (parts per million by volume) in 2007 compared to 290 in 1880, about 280 before the Industrial Revolution began, and never more than 300 during the 420,000 years before the Industrial Revolution.
Recap of Situation Today:
Atmospheric Carbon Dioxide
- is 35% higher than when Industrial Revolution began
- is higher than any period in last 420,000 years
- is primary cause of temperature increase (not the other way around)
If no action is take to abate greenhouse gases --
- may rise by 2-6°C ... 4-11°F in the 21st Century
which will be higher than at any point
- in the last 10,000 years (Holocene Epoch) when we developed the foods that support us and the fruits of our civilization
- since the middle Pliocene Epoch three million years ago when sea level may have been 25 meters or 80 feet higher than today
The temperature increase was less dramatic -- 0.85°C (1.53°F) -- but much of the carbon dioxide added in this period will remain in the atmosphere and continue to warm the Earth for centuries to come.
The NASA report said that the "only viable explanation for warming after 1950 is an increase in greenhouse gases."
So what happened to Milankovitch?
Before the Industrial Revolution and the huge increase in population that followed, temperature change over the long term was largely caused by change in sunlight explained by the Milankovitch Cycles. Temperature change influenced change in greenhouse gases, which in a so-call positive feedback mechanism then accelerated the temperature change.
Now, however, the recent surge in greenhouse gas emissions has trumped the Milankovitch Cycles. The number one cause of the current upward trend in Earth's temperature is the increase in atmospheric carbon dioxide, not an increase in sunlight. And the increase in carbon dioxide is caused primarily by human activity, notably the burning of fossil fuel (oil, natural gas and coal), not by the temperature increase -- although in another positive feedback mechanism the temperature increase may help to increase the amount of carbon dioxide in the atmosphere.
4. Is the U.S. Government Concerned about Temperature Change?
James Hansen took global warming public in his Congressional testimony in the summer of 1988, but Congress has never taken action, and neither has the Executive Branch. Indeed, the last three Administrations (Bush, Clinton, Bush) have all tried to muzzle him.
In its 2007 Climate Change Synthesis Report, the Intergovernmental Panel on Climate Change (IPCC), which shared that year's Nobel Peace Prize with Al Gore, projected temperature increases in the 21st Century of from 2 to 6°C (4 to 11°F) if no action is taken beyond what little has already been taken to mitigate greenhouse gas emissions.
Should we be worried? Should our grandchildren?
According to Hansen, "if further global warming reaches 2 or 3 degrees Celsius, we will likely see changes that make Earth a different planet than the one we know. The last time it was that warm was ... about three million years ago, when sea level was estimated to have been about 25 meters (80 feet) higher than today."
In April 2008, Hansen and seven other scientists from as many universities and institutions submitted an abstract to Science entitled Target atmospheric CO2: Where should humanity aim? Their conclusion: "If humanity wishes to preserve a planet similar to the one on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm [in 2008] to at most 350 ppm."
When Bill McKibben, longtime global warming activist and author of , read these words, he started a new grassroots climate movement called 350.org "to make sure everyone knows the target so that our political leaders feel real pressure to act." The End of Nature
He admits it's a bit of a Hail Mary. The last year when carbon dioxide was down to 350 was 1987. What if we don't make it?
"People will doubtless survive on a non-350 planet," McKibben writes, "but those who do will be so preoccupied, coping with the endless unintended consequences of an overheated planet, that civilization may not."
Jim Hansen's Web Site
All comments, questions and views are welcome!