Global warming: What the science tells us

Thomas R. Karl
Thomas R. Karl (NOAA (National Oceanic and Atmospheric Admninistration))
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Thomas R. Karl
Director, National Climatic Data Center
Friday, December 11, 2009; 11:00 AM

LATEST NEWS: Key Copenhagen group releases draft climate plans

Two top Obama administration officials arrived this week at the U.N.-sponsored climate talks offering both diplomacy and a tough line: The United States is willing to be a full partner in fighting climate change, but the real problem is with China and the developing world.

At a critical time, the uproar over stolen e-mails suggesting scientists suppressed contrary views about climate change has emboldened skeptics -- including congressional Republicans looking to scuttle President Barack Obama's push for mandatory reductions in greenhouse gases.

The climate skeptics gained political momentum when former Republican vice presidential candidate Sarah Palin said Obama should boycott the negotiations in Denmark and "not be a party to fraudulent scientific practices" -- a clear reference to the purloined e-mails from computers belonging to scientists at a British climate research center.

Thomas R. Karl, director of the National Climatic Data Center at NOAA (National Oceanic and Atmospheric Administration) and recently returned from Copenhagen was online Friday, Dec. 11, at 11 a.m. ET to discuss the scientific aspects of climate change.


Thomas R. Karl: Hi. My name is Tom Karl and I am the Director of NOAA's National Climatic Data Center in Asheville, North Carolina and I am also NOAA's lead for Climate Services. I've just returned from the climate conference in Copenhagen, (COP 15 where approximately 15,000 people will gather over the two-week period to work together towards solving the problem of human-caused climate change. There, I gave two presentations, one on changes in climate extremes and one on climate change impacts in the U.S. - including both the impacts. I'm here today to talk about the science behind global climate change, to answer questions about what we know, what the data tells us, and to dispel misconceptions and myths.


Germantown, Md.: Good Afternoon,

A lot has been made about the e-mails from East Anglia, (especially in Washington, D.C.). It's difficult to assess the true scope of the issue because of the plainly partisan bias at play in the government. Could you please explain what exactly the e-mails mean for the scientific community at work on climate issues, and also explain why East Anglia university is important in this debate, if it is at all. (If it had been MIT, or Harvard, how would that have affected the debate?) Thank you!

Thomas R. Karl: These emails do nothing to undermine the very strong scientific consensus that the earth is warming and that human activity is largely responsible.


Washington, D.C.: Everyone goes on about the Arctic ice melting and causing catastrophic flooding in coastal areas. But since most of the Arctic ice is floating ice, wouldn't this have little to no effect on sea levels? Think of an ice cube in a glass of water. When the ice cube melts, the level of water has not changed. So why the panic?

Thomas R. Karl: You are absolutely correct. Melting of Arctic sea ice does not change global sea level. Sea-level rise is caused by increasing ocean temperatures, which expands the volume of water in the ocean. Additionally, melting of the world's glaciers contributes to sea-level rise, and of particular concern is the recently observed melting of major ice sheets, including Greenland.

Sea-level rise has been accelerating over the past two decades compared to the previous 100 years of observed data. This trend is expected to continue with potential large additional contributions of melting from the major ice sheets.


Amherst, Mass.: Why isn't the raw temperature measurement data set/s freely available to anyone who wants to do own calculations?

Thomas R. Karl: All original NOAA data are freely available to anyone who requests it.

All of the data that NOAA uses to develop our time series are openly available. NOAA operates 3 National Data Centers and 5 World Data Centers. Part of their mission is to make climate-related data available to all users.

Data are available at, phone: 828-271-4800.

_______________________ National Climatic Data Center


Potomac, Md.: Did NOAA/GHCN "homogenize" their climate data so that temperature declines became temperature increases as described in the following Web site?

NOAA/GHCN "homogenization" falsified climate declines into increases (Hot Air, Dec. 9)

Why were similar "fudge factors" included in the CRU computer code as described in the following website?

Revenge of the Computer Nerds (American Thinker, Dec. 9)

Thomas R. Karl: All NOAA data that are used to calculate global temperature change are subject to quality assurance processing algorithms. These algorithms are also made available for testing upon request and are peer-reviewed in the open scientific literature. Changes in the algorithms are documented, and that documentation is available. NOAA scientists work collaboratively with other scientists who are experts in the field, e.g. state climatologists in the USA and World Meteorological Organization Expert Teams. Over the years many scientists have collaborated with NOAA to provide data, insight, and oversight in the quality assurance process.

For global climate monitoring, NCDC developed a global blended temperature data set, which combines land surface data.

All data are provided online at: .

Information on the methods used in developing these datasets is available in the following journal articles, which discuss the adjustments for systematic bias in the data.

Smith, T.M., and R.W. Reynolds, 2003: Extended Reconstruction of Global Sea Surface Temperatures Based on COADS Data (1854-1997). Journal of Climate, 16, 1495-1510.


Smith, T.M., and R.W. Reynolds, 2004: Improved Extended Reconstruction of SST (1854-1997). Journal of Climate, 17, 2466-2477.


Smith, T.M., R.W. Reynolds, Thomas C. Peterson, and Jay Lawrimore, 2008: Improvements to NOAA's Historical Merged Land-Ocean Surface Temperature Analysis (1880-2006). Journal of Climate,21, 2283-2296.


Silver Spring, Md.: Hello,

Many people imply that the CRU temperature data are the exclusive or principal basis for climate change predictions. Please identify some key studies that do not rely heavily on CRU data, and their conclusions. Thanks.

Thomas R. Karl: Hi there - thanks for the question. In fact, there are other global temperature datasets that are calculated by other institutions. For example, NASA calculates an independent global temperature dataset, as does NOAA (here at National Climatic Data Center). The analysis techniques for each of these datasets are all independent of each other and yet they all come to the same conclusion: that global warming is unequivocal. In addition to direct temperature measurements, there are many other lines of evidence that we can use to gauge the amount of warming. For example, melting glaciers, reduced Northern Hemisphere snow cover extent, increased ocean heat content and many more indicators help us understand recent climate change. See a web page on other global climate change indicators:


Richmond, Va.: Mr. Karl,

re your answer to the first question, can you elaborate? What is the role of East Anglia and what was their motivation in deleting e-mails, "tricking" statistics and stonewalling Freedom of Information requests?

Thomas R. Karl: East Anglia is a respected research institution. It is critical to point out that datasets from other research centers show the same climate trends as the data from East Anglia. The conclusions of the IPCC reports are based on many data sets, in addition to the Climatic Research Unit (CRU) including data from NOAA and NASA. Each of those datasets show virtually the same multi-decadal climate trends.


Freising, Germany: I've read the viewpoint that combating global warming would be more expensive than adjusting to a new climate. I find this hard to believe, because as oil and gas became scarce and astronomically expensive, the austerity measures will appear draconian compared to measures to find new energy sources.

What are thoughts on combating versus adjusting?

Thomas R. Karl: Thanks - the bottom line is that we will need both mitigation action (reducing carbon dioxide concentrations) as well as adaptation measures (being able to adjust to a new climate). In our recent U.S. Global Change Research state of knowledge report on climate impacts in the U.S. (, one of the major conclusions we reached was that earlier reductions in emissions will be more effective than the same reductions undertaken later. We also outlined the difference in expected impacts between higher and lower emissions scenarios. Many of the possible impacts under a higher emissions scenario are extreme and would pose difficulties for people in adapting to them. We also looked at some emerging examples of adaptation across the U.S. Clearly, there is a lot more to learn about how to adapt to a changing climate.


New Orleans, La.: Is there a strong scientific consensus on exactly what human activities are responsible for the warming earth and the proportion caused by each activity?

Thomas R. Karl: Yes, there is strong scientific consensus that humans are responsible for most of the warming observed since the 1950s. The Intergovernmental Panel on Climate Change Fourth Assessment Report concludes that it is very likely that most of the warming since 1950 is due to human influences on the climate.

We actually know why humans are causing climate change. This is based on extensive modeling studies and climate analyses. This has been reported upon by every major scientific society, global governments, National Academies of Sciences (including our own), as well as our own government reports which transcend several administrations.


Washington, D.C.: Dear Dr. Karl: Thank you for participating in this chat. Are you and other scientists alarmed at all about the surveys showing 'fewer' Americans now believe global warming is happening at all, or that it is caused by man than only two years ago?

Is there a way for scientists to seize the agenda back from the well-funded "skeptics?" I think you need a Carl Sagan type of spokesman.

Thomas R. Karl: Hi, thanks for this question. As you may know, parts of the United States have experienced prolonged cooler-than-average conditions at times this year. At the same time, worldwide temperatures continue to stay very warm, compared to the last couple of centuries. We are trying to understand how these local and short-term experiences may affect peoples' perceptions of global climate changes. We are working with the social science community to help us better understand.


Rockville, Md.: How does one decide if warming is good or bad?

Thomas R. Karl: It is the rate of change that challenges the optimal functioning of ecosystems on which we depend, including life in the sea and the enormous infrastructure that we and other nations have developed.

For example, our transportation system, including airports, docks and ports, roads, and rail are all built upon a stable climate using past climate information to optimize their operations. When they are flooded, or when pavement or rails buckle under extreme heat, then this has a decidedly negative impact on our Nation's commerce.


Lake Ozark, Miss.: Please comment on the statement below by Ian Plimer:

"Carbon dioxide has an effect on the atmosphere and it has an effect for the first 50 parts per million and once it's done its job then it's finished and you can double it and quadruple it and it has no effect because we've seen that in the geological past, and we've seen it in times gone by when the carbon dioxide content was 100 times the current content. We didn't have runaway global warming, we actually had glaciation, so there's immediately a disconnect. So carbon dioxide is absolutely vital for living on earth; it's plant food, all of life lives off carbon dioxide. To demonize it shows that you don't understand school child science." -Ian Plimer, interviewed on ABNNewswire, June 2009

Thomas R. Karl: Higher carbon dioxide levels generally cause plants to grow larger, including weeds and noxious plants (e.g., poison ivy). For some crops, this is not necessarily a benefit because they are often less nutritious, with reduced nitrogen and protein content.

In addition to carbon dioxide's heat-trapping effect, the increase in its concentration in the atmosphere is gradually acidifying the ocean. About one-third of the carbon dioxide emitted by human activities has been absorbed by the ocean, resulting in a decrease in the ocean's pH. Since the beginning of the industrial era, ocean pH has declined demonstrably and is projected to decline much more by 2100 if current emissions trends continue. Further declines in pH are very likely to continue to affect the ability of living things to create and maintain shells or skeletons of calcium carbonate.


Rockville, Md.: My family once lived on a cotton farm in Texas where our growing season was just a few days (or weeks at best) longer than what we needed for a crop. How is warmer and a longer growing season bad for cotton crops? Perhaps all we need to do is have a larger Canadian wheat crop. Is all this gloom about warmer justified? Or just selective?

Thomas R. Karl: Many crops show positive responses to elevated carbon dioxide and low levels of warming, but higher levels of

warming often negatively affect growth and yields. Increased pests, water stress, diseases, and weather extremes

will pose adaptation challenges for crop and livestock production.

To learn more about this important topic, please go to:


Atlanta, Ga.: So climate change could result in higher sea levels which may negatively impact coastal or low-lying lands (islands, beaches, etc). But couldn't such warming also positively impact areas that have previously suffered extreme drought through changing storm tracks, or by expanding the arable lands - such that tropical fruits or plants will sustain in areas such as Virginia, Tennessee, and other areas that can't sustain such food - point being, with every negative possible impact of warming, aren't there just as many positive possible impacts?

Thomas R. Karl: A changing climate is not the same as a stable climate where one can rely on having an equal number of favorable and unfavorable years for agriculture.

For a changing climate, these odds are continually being changed. For example, climate change is expected to increase drought and other extremes. Changing storm tracks would negatively impact areas the currently receive most of their precipitation from these storms. The expectation is that the sub-tropical arid areas likely will expand into higher latitudes resulting in drier conditions in these areas decreasing agricultural productivity. Also, increasing evaporation due to increasing temperatures in the mid-continental regions will outweigh any increases in precipitation resulting in drier conditions in the growing season.


Washington, D.C.: I have read two things from opposite sides of the debate: 1. There has been global cooling for the last 10 years, and 2. The last decade has been the warmest on record. Obviously someone is playing with words or with data. Could you clarify? Thanks.

Thomas R. Karl: Global temperatures this decade have indeed been the warmest on record, not just according to NOAA, but according to all of the major global institutions that monitor global temperature change. Some months ago, a narrative began that claimed that we've seen cooling since a very warm 1998. This simply isn't true, by several measures. 1998 was an exceptionally warm year, fueled in part by a strong El Nino. According to our records, it is still the second-warmest year on record, next to 2005. However, eight of the nine warmest years on record have all occurred this decade. In three weeks, when 2009 comes to a close, nine of the ten warmest years will have occurred this decade. In fact, our NOAA data show continued warming since 2000.


Rockville, Md.: For the sake of discussion, suppose that it is warming. Also presume that an ice age is caused by astronomical factors; such as orbits, solar mean radiation and dust in space, etc. Will a warming cycle theory based on mechanisms of the atmosphere prove to be accurate and still not predict an ice age or a cooling trend that is not based in the atmosphere.

Or do we need more factors to be included in our study?

I see a place where (1) yes it is getting warmer and that theory is correct, but (2) it is not all we need to know about temperatures in the future.

Are we too limited in our work?

Thomas R. Karl: Excellent question. Climatologists are studying all aspects of climate change on all time scales, including scales of millions of years. You are correct that the ice ages came and went due to orbital factors, but other climate feedbacks were involved as well (such as CO2 concentration going down moving into an ice age and going up at the end of an ice age, likely because of release of carbon and methane trapped in permafrost).

The bottom line is that climate changes for a reason. The ice age due to changes in Earth's orientation to the Sun. The warming over the last 50 years due to man-made changes in the atmosphere.


Winter Park, Fla.: How does our current climate compare with the Middle Ages and Roman warm periods?

Thomas R. Karl: Parts of the world were warmer during the Middle Ages, particularly northern Europe, Iceland and Greenland. Several decades ago, scientists thought the whole world was warmer than it is now. But as new data sources come in from tree rings in the U.S., corals in the tropical oceans, etc. we have learned that the average temperature of the world was likely to have been cooler during the "Medieval Warm Period" than it is now.


Thomas R. Karl: Well, I want to thank you all for your questions. This was really a good exchange, with great questions on an important topic of our time. What I hope we all got out of this is that the Earth is warming at an unprecedented pace, that there is ample evidence for that, and that human activity is largely to blame. It is now up to the United Nations Framework Convention, 15th Conference of the Parties in Copenhagen, to help us solve this problem.

Some additional resources on the science of climate change include:


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