Climate Skeptics Questions Answered – Columbia University

Submitted by amyguinan on September 28, 2010 – 6:14am

Columbia Climate Center at the Earth Institute, Columbia University, has released a paper which examines as many as possible of the major skeptic claims and counter-claims related to global climate changed caused by increase carbon in the atmostphere. The resulting paper makes for a very accessible study of the science related to climate change projections and is a well-researched tool for connecting with and explaining the challenge that is climate change.   Claims and counter-claims including climate projection models, conspiracy accusations, past warming cycles on Earth, and carbon sequestration are explored.



Global average temperatures have not risen since 1998. In fact, the decade of 2000 to 2009 is the warmest since measurements have been made. Multiple factors affect global average temperatures, including the long-term warming trend from GHGs. This time-varying interaction of climate drivers can lead to periods of relatively stable temperatures interspersed with periods of warming. The anomalously high global average temperatures in 1998 associated with the El Niño have been followed by comparably high values that reflect a combination of long-term warming and shorter-term natural variability. Periods of relatively constant temperature are not evidence against global warming.
Climate researchers are engaged in a conspiracy: global warming is a hoax. There is no evidence that scientists have engaged in alleged conspiracies. Three investigations discerned no scientific misconduct in emails stolen from University of East Anglia’s Climatic Research Unit. It has been claimed that the decline in the number of weather stations in the global network since the 1990s was due to purposeful removal, but there is no evidence to support this; furthermore, the reduction in number of stations reporting data has introduced no detectable bias in the trend of the global average temperature anomaly.  The IPCC reports undergo significant scrutiny, but as is inevitable in a 3000-page document, that scrutiny sometimes fails to detect errors. The few errors identified in the latest IPCC report were primarily in referencing and not in content.  Their existence does not support a conspiracy to misrepresent climate research.
Climate models are defective and therefore cannot provide reliable projections of future climate trends. We do not rely only on models for our understanding of the effect of greenhouse gases on climate. Theory (i.e. the physics and chemistry of the planet’s atmosphere and ocean) and observations are the foundation of our ability to understand climate and to assess and quantify forcing and impacts. Models represent the most formal way in which to project and quantify future conditions. Despite well known limitations to climate models such as the uncertainties of clouds, aerosols, and spatial resolution, climate models are increasingly able to reproduce a range of physical processes and feedbacks. They unanimously predict warming with increasing greenhouse gases of a magnitude consistent with estimates independently derived from observed climate changes and past climate reconstructions.
The greenhouse gas signature (tropospheric hot spot) is missing. Climate models predict that GHGs cause cooling in the stratosphere and warming at the surface and throughout the troposphere. Observations are consistent with these predictions.  Furthermore, new measurements in the tropics suggest greater warming in the upper troposphere than at the surface, as predicted by the models. Although the tropospheric hot spot (signature) is not unique to greenhouse gas forcing, the new observational data lend support to climate simulations.
The Medieval Warm Period was just as warm as, or warmer than, today. Northern hemisphere temperatures in the Medieval Warming Period (MWP) may have been comparable to today, but the estimates have high uncertainty because there are so few records and spatial coverage is spotty. However, a MWP warmer than the last decade does not challenge the case for anthropogenic warming.
Carbon dioxide levels increase after temperatures rise in the  ice core records. The correlation of records of atmospheric CO2 and Antarctic temperature over the past 800,000 years indicates that CO2 amplified the warming attributed to variability in Earth’s orbit in the transition out the ice ages. Different processes can and do affect climate concurrently.
Earth’s climate is driven only by the sun. While Earth’s climate is undoubtedly driven by the sun, the sun is not the only factor that determines climate. The observations of warming since the second half of the 20th century cannot be explained by solar activity because it has been decreasing. Sunspots, areas of the sun’s surface that appear darker in a telescope, have been observed since the invention of telescopes in 1610. Although climate predictions from sunspots have long been attempted, the predictions have not held up.
Water vapor is the most prevalent greenhouse gas. Water vapor plays a primary role in the natural greenhouse effect, but that does not diminish the impact of CO2-induced warming.  The concentration of water vapor is a positive feedback: as the earth warms, the atmosphere can hold more water vapor, which, in turn, warms it further.
CO2 in the atmosphere is already absorbing all of the infrared radiation that it can. Carbon dioxide in the atmosphere at pre-industrial levels already causes substantial radiation absorption.  Anthropogenic increases in carbon dioxide concentration cause further absorption (over a broader range of wavelengths) and change the height distribution of the absorption.  These effects generate warming and are fully incorporated in current climate models.
Climate sensitivity is overestimated in current climate models. Climate sensitivity is defined as the change in global mean temperature that occurs in response to a doubling of atmospheric carbon dioxide. Values between 2 and 4.5oC are consistent with our understanding of forcing and past and present climate shifts. Lower estimates tend to disregard feedbacks, like water vapor, and delays associated with slower earth system components, like the ocean. Furthermore, sensitivity values below 2.5oC cannot explain the observed climate changes of the past.
Increasing carbon dioxide will stimulate plant growth and improve agricultural yield. Plant growth is stimulated by increased levels of CO2 under equal conditions of temperature and availability of water and nutrients. However, altered growing conditions due to projected shifts in climate will likely counter the fertilization effect in large parts of the already food-insecure world. Experiments to quantify the impact of CO2 enrichment indicate that increased growth does not occur for all plant species and that nutritional content of crops is sometimes negatively impacted.
Human society and natural systems have adapted to past climate change. Past climate changes have often been accompanied by migration, war, and disease. The growing human population will inevitably make environmental change more disruptive in the future, even in the face of increased technological prowess.

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