Are the oceans warming?

Sea surface temperatures are shown in the figure on the right (data provided by NOAA). On the whole, they show a similar trend to that of global land surface temperatures.
The oceans are also warming at depth. There are sufficient data from the upper 3,000 m to provide a global estimate. In March 2000, NOAA reported that this layer had warmed on average by 0.06°C over the past 40 years. Most of this warming occurred in the top 300m, which warmed by around 0.3°C.
These data show that heat is entering the ocean from the surface and spreading downwards. The pattern of warming is remarkably similar to predictions from climate models, suggesting that the cause of the warming is primarily greenhouse gases.

Greenhouse Gas

Gases produced from human activities that trap solar radiation and thus contribute to climate change and the destruction of the ozone layer. These include:

• CO2 Carbon Dioxide


• CH4 Methane


• HFCs Hydrofluorocarbons (a class of several gasses)


• N2O Dinitrogen Oxide


• PFCs Perfluorocarbons (a class of several gasses)


• SF6 Sulfur Hexafluoride

Climate forcings, sensitivity, response time and feedbacks

A climate forcing is an imposed perturbation of the Earth's energy balance. If the sun brightens, that is a positive forcing that warms the Earth. Aerosols (fine particles) blasted by a volcano into the upper atmosphere reflect sunlight to space, causing a negative forcing that cools the Earth's surface. These are natural forcings. Human-made gases and aerosols are also important forcings.

Climate sensitivity is the response to a specified forcing, after climate has had time to reach a new equilibrium, including effects of fast feedbacks. A common measure of climate sensitivity is the global warming caused by a doubling in atmospheric CO2 concentration. Climate models suggest that doubled CO2 would cause 3 °C global warming, with an uncertainty of at least 50%. Doubled CO2 is a forcing of about 4 W/m2, implying that global climate sensitivity is about 3/4 °C per W/m2 of forcing.

Climate response time is the time needed to achieve most of the climate response to an imposed forcing, including the effects of fast feedbacks. The response time of the Earth's climate is long, at least several decades, because of the thermal inertia of the ocean and the rapid mixing of waters within the upper few hundred meters of the ocean. Climate sensitivity and response time depend upon climate feedbacks, which are changes in the planetary energy balance induced by the climate change that can magnify or diminish climate response. Feedbacks do not occur immediately in response to a climate forcing; rather, they develop as the climate changes.

Fast feedbacks come into play quickly as temperature changes. For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.

Slow feedbacks, such as ice sheet growth and decay, amplify millennial climate changes. Ice sheet changes can be treated as forcings in evaluating climate sensitivity on time scales of decades to centuries.

What can Paleoclimatology tell us about climate change relevant to society in the future?

To understand and predict changes in the climate system, we need a more complete understanding of seasonal to century scale climate variability than can be obtained from the instrumental climate record. The instrumental temperature record indicates that the Earth has warmed by 0.5°C (0.9°F) from 1860 to the present. However, this record is not long enough to determine if this warming should be expected under a naturally varying climate, or if it is unusual and perhaps due to human activities. Paleoclimatic proxy data can be used to extend climate records and provide a longer time frame (hundreds to tens of thousands of years) for evaluating the warming of the last 140 years. The cause of global warming over the last century remains a heated debate with significant economic and societal implications. Many scientists attribute the current global warming to the enhancement of the greenhouse effect by human activities. Other scientists have suggested that other factors not affected by humans, such as changes in the number and size of volcanic eruptions or an increase in the sun's output (such phenomena are referred to as climate forcings), are responsible. A paleoclimate perspective provides information about long term changes in different climate forcings that may be the underlying cause of the observed climate change. An analogy of how paleoclimatic data improves our understanding of climate can be explained in terms of the stock market. Stock market analysts use longer term trends (one, two, three, or six months) in the stock market indexes (DOW, NASDAQ, etc.) rather than depending on changes from one day to the next or over a week to predict what the market will do next (i.e., Bull or Bear Market). In much the same way, the paleoclimate perspective allows us to evaluate climate change many decades and centuries into the past, in order to develop a more reliable estimate of how climate may change in the future.


The paleoclimate perspective can help us answer many questions, including...

• Is the last century of climate change unprecedented relative to the last 500, 2000, and 20,000 years?
• Do recent global temperatures represent new highs, or just part of a longer cycle of natural variability?
• Is the recent rate of climate change unique or commonplace in the past?
• What does it mean if the last century is unprecedented in terms of warming?
• Can we find evidence in the paleoclimate record for mechanisms or climate forcings that could be causing recent climate change?

What is Climate?

Climate is the weather pattern we expect over the period of a month, a season, a decade, or a century. More technically, climate is defined as the weather conditions resulting from the mean, or average, state of the atmosphere-ocean-land system, often described in terms of "climate normals" or average weather conditions. Climate Change is a departure from the expected average weather or climate normals.

Greenland's Ice Sheet Melting

In a recent study by researchers from NASA's Goddard Space Flight Center shows that Greenland's ice sheet, about 8% of the Earth's grounded ice (Antarctica possessing 91% of land ice), is losing ice mass. A NASA high-tech aerial survey shows that more than 11 cubic miles of ice is melting along Greenland's coasts yearly, accounting for 7% of the annual global sea level rise. Measurements over the last century suggest that sea level has risen 9 inches, enough to cause flooding in low-lying areas, when a storm occurs. Sea level increase could worsen, if the present trend continues, says William Krabill, lead author of the NASA study.

Panel Supports a Controversial Report on Global Warming

An influential and controversial paper asserting that recent warming in the Northern Hemisphere was probably unrivaled for 1,000 years was endorsed Thursday, with a few reservations, by a panel convened by the nation's pre-eminent scientific body.

The panel said that a statistical method used in the 1999 study was not the best and that some uncertainties in the work "have been underestimated," and particularly challenged the authors' conclusion that the 1990's were probably the warmest decade in a millennium.
But in a 155-page report, the 12-member panel convened by the National Academies said "an array of evidence" supported the main thrust of the paper. Disputes over details, it said, reflected the normal intellectual clash that takes place as science tests new approaches to old questions.
The study, led by Michael E. Mann, a climatologist now at Pennsylvania State University, was the first to estimate widespread climate trends by stitching together a grab bag of evidence, including variations in ancient tree rings and temperatures measured in deep holes in the earth.
It has been repeatedly attacked by Republican lawmakers and some industry-financed groups as built on cherry-picked data meant to create an alarming view of recent warming and play down past natural warm periods.
At a news conference at the headquarters of the National Academies, several members of the panel reviewing the study said they saw no sign that its authors had intentionally chosen data sets or methods to get a desired result.
"I saw nothing that spoke to me of any manipulation," said one member, Peter Bloomfield, a statistics professor at North Carolina State University. He added that his impression was that the study was "an honest attempt to construct a data analysis procedure."
More broadly, the panel examined other recent research comparing the pronounced warming trend over the last several decades with temperature shifts over the last 2,000 years. It expressed high confidence that warming over the last 25 years exceeded any peaks since 1600. And in a news conference here on Thursday, three panelists said the current warming was probably, but not certainly, beyond any peaks since the year 900.
The experts said there was no reliable way to make estimates for surface-temperature trends in the first millennium A.D.
In the report, the panel emphasized that the significant remaining uncertainties about climate patterns over the last 2,000 years did not weaken the scientific case that the current warming trend was caused mainly by people, through the buildup of heat-trapping greenhouse gases in the atmosphere.
"Surface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting the conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence," the report said.
The 1999 paper is part of a growing body of work trying to pull together disparate clues of climate conditions before the age of weather instruments.
The paper includes a graph of temperatures in the Northern Hemisphere that gained the nickname "hockey stick" because of its vivid depiction of a long period with little temperature variation for nearly 1,000 years, followed by a sharp upward hook in recent decades.
The hockey stick has become something of an environmentalist icon. It was prominently displayed in a pivotal 2001 United Nations report concluding that greenhouse gases from human activities had probably caused most of the warming measured since 1950. A version of it is in the Al Gore documentary "Inconvenient Truth."
Senator James M. Inhofe, Republican of Oklahoma, and Representative Joe L. Barton, Republican of Texas, have repeatedly criticized the Mann study, citing several peer-reviewed papers challenging its methods.
The main critiques were done by Stephen McIntyre, a statistician and part-time consultant in Toronto to minerals industries, and Ross McKitrick, an economist at the University of Guelph in Ontario.
They contended that Dr. Mann and his colleagues selected particular statistical methods and sets of data, like a record of rings in bristlecone pine trees, that were most apt to produce a picture of unusual recent warming. They also complained that Dr. Mann refused to share his data and techniques.
On his Web log, climateaudit.org, on Thursday, Mr. McIntyre said the panel's report seemed to have "two completely distinct personalities," upholding specific criticisms of Dr. Mann's methods, but still positing it was plausible that recent warming exceeded any warm periods for 1,000 years.
In an interview, Dr. Mann expressed muted satisfaction with the panel's findings. He said it clearly showed that the 1999 analysis had held up over time.
But he complained that the committee seemed to forget about the many caveats that were in the original paper. "Even the title of the paper on which all this has been based is as much about the caveats and uncertainties as it is about the findings," he said.
The paper, published in the journal Geophysical Research Letters, was called "Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties and Limitations."
Raymond S. Bradley, a University of Massachusetts geoscientist and one of Dr. Mann's co-authors, said that the caveats were dropped mainly as the graph was widely reproduced by others. (The other author of the 1999 paper was Malcolm K. Hughes of the University of Arizona.)
The report was done at the request of Representative Sherwood Boehlert, the New York Republican who is chairman of the House Science Committee, who called last November for a review of the 1999 study and related research to clear the air.
In a statement, Mr. Boehlert, who is retiring at the end of the year, expressed satisfaction with the results, saying, "There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change — which doesn't rest primarily on these temperature issues, in any event — or any doubts about whether any paper on the temperature records was legitimate scientific work."
Critics of the paper remain unconvinced.
A separate panel of statisticians is dissecting Dr. Mann's data and papers for the House Committee on Energy and Commerce, a spokesman for the chairman, Mr. Barton, said.

What Causes the Greenhouse Effect?

Life on earth depends on energy from the sun. About 30 percent of the sunlight that beams toward Earth is deflected by the outer atmosphere and scattered back into space. The rest reaches the planet’s surface and is reflected upward again as a type of slow-moving energy called infrared radiation.
As infrared radiation is carried aloft by air currents, it is absorbed by “greenhouse gases” such as water vapor, carbon dioxide, ozone and methane, which slows its escape from the atmosphere.
Although greenhouse gases make up only about 1 percent of the Earth’s atmosphere, they regulate our climate by trapping heat and holding it in a kind of warm-air blanket that surrounds the planet.

This phenomenon is what scientists call the "greenhouse effect." Without it, scientists estimate that the average temperature on Earth would be colder by approximately 30 degrees Celsius (54 degrees Fahrenheit), far too cold to sustain our current ecosystem.

The Greenhouse Effect

Over the last 400,000 years the Earth's climate has been unstable, with very significant temperature changes, going from a warm climate to an ice age in as rapidly as a few decades. These rapid changes suggest that climate may be quite sensitive to internal or external climate forcings and feedbacks. As can be seen from the blue curve, temperatures have been less variable during the last 10 000 years. Based on the incomplete evidence available, it is unlikely that global mean temperatures have varied by more than 1°C in a century during this period. The information presented on this graph indicates a strong correlation between carbon dioxide content in the atmosphere and temperature. A possible scenario: anthropogenic emissions of GHGs could bring the climate to a state where it reverts to the highly unstable climate of the pre-ice age period. Rather than a linear evolution, the climate follows a non-linear path with sudden and dramatic surprises when GHG levels reach an as-yet unknown trigger point.

September 25, 2007 NASA-Melting Arctic sea ice has shrunk to a 29-year low, significantly below the minimum set in 2005, according to preliminary figures from the National Snow and Ice Data Center, part of the University of Colorado at Boulder. NASA scientists, who have been observing the declining Arctic sea ice cover since the earliest measurements in 1979, are working to understand this sudden speed-up of sea ice decline and what it means for the future of Earth's northern polar region.
At the end of each summer, the sea ice cover reaches its minimum extent and the ice that remains is called the perennial ice cover, which consists mainly of thick multi-year ice floes. The area of the perennial ice has been steadily decreasing since the satellite record began in 1979, at a rate of about 10% per decade. But the 2007 minimum, reached around Sept. 14, is far below the previous record made in 2005 and is about 38% lower than the climatological average. This data visualization shows the annual sea ice minimum from 1979 through 2007.

The Advanced Microwave Scanning Radiometer (AMSR-E) is a high-resolution passive microwave Instrument on NASA's Aqua satellite. AMSR-E provides a remarkably clear view of sea ice dynamics in greater detail than has ever been seen before. Researchers use this information to study polar bear habitats, plan expeditions to the ice, and to study the interactions between the ocean and sea ice from season to season. This data visualization shows Arctic sea ice from Jan. 1, 2007 to Sept. 16, 2007.
Because Arctic ice cover varies so much year to year, it can be dangerous to look at any one year and draw too much of a conclusion from it," said Waleed Abdalati, head of Goddard's Cryospheric Sciences Branch. "But this year, the amount of ice is so far below that of previous years that it really is cause for concern. The trend in decreasing ice cover seems to be getting stronger and stronger as time goes on."NASA developed the original capability to observe the extent and concentration of sea ice from space using passive microwave sensors. More recently, NASA launched an advanced microwave instrument in 2002 -- the Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite -- that provides a view of sea ice dynamics in greater detail than has ever been seen before. Researchers use this information to study polar bear habitats and the unique movements of sea ice from season to season. AMSR-E is a joint project of NASA and the National Space Development Agency of Japan.

In September 2007, the Northwest Passage was ice-free for the first time since satellite records began. The passage is a direct route from Europe to Asia for ships traveling through the Arctic. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra satellite captured this image of the ice-free Northwest Passage on Sept. 15, 2007.
Current satellites, however, can map sea ice in two dimensions, but it is much more difficult to find out how the thickness of the ice contributes to the change in the total volume of the ice. NASA's ICESat spacecraft (Ice, Cloud, and land Elevation Satellite), launched in 2003, with the primary goal of determining how much ice sheets are contributing to sea-level rise. ICESat is also collecting data that enables scientists to make estimates of sea ice thickness with unprecedented detail."What we need to truly understand the interaction of the ice, ocean and atmosphere in the Arctic is sea ice thickness information," said Abdalati. "The new capability we have with ICESat is expected to be extended into the next decade based on recent recommendations by the National Research Council for a follow-on mission. Ultimately, like the 29-year record we have now of sea ice cover, a long-term ice thickness record will help scientists understand these complex interactions and what the changes in the ice cover will mean to the ecology of the Arctic and to life on Earth."NASA has been observing sea ice from space since the 1970s, beginning with the Electricallly Scanning Microwave Radiometer (ESMR), Scanning Multichannel Microwave Radiometer (SSMR) and Special Sensor Microwave/Imager (SSM/I) sensors on the US Defense Meteorological Space Program (DMSP) satellites, and now with the AMSR-E instrument on NASA's Earth Observing System/Aqua satellite. Data collected by these instruments have been instrumental in shaping public policy and international perspectives on the Arctic.