We discuss climate models a lot, and from the comments here and in other forums it's clear that there remains a great deal of confusion about what climate models do and how their results should be interpreted. This post is designed to be a FAQ for climate model questions - of which a few are already given. If you have comments or other questions, ask them as concisely as possible in the comment section and if they are of enough interest, we'll add them to the post so that we can have a resource for future discussions. (We would ask that you please focus on real questions that have real answers and, as always, avoid rhetorical excesses).
(more…)

We've often been asked to provide a one stop link for resources that people can use to get up to speed on the issue of climate change, and so here is a first cut. Unlike our other postings, we'll amend this as we discover or are pointed to new resources. Different people have different needs and so we will group resources according to the level people start at.

For complete beginners:

NCAR: Weather and climate basics
Oxford University: The basics of climate prediction
Pew Center: Global Warming basics
Wikipedia: Global Warming
NASA: Global Warming update
National Academy of Science: Understanding and Responding to Climate Change
Encyclopedia of Earth: Climate Change Collection
Global Warming FAQ (Tom Rees)

Those with some knowledge:

The IPCC AR4 Frequently Asked Questions (here) is an excellent start. That covers:

• What Factors Determine Earth's Climate?
• What is the Relationship between Climate Change and Weather?
• What is the Greenhouse Effect?
• How do Human Activities Contribute to Climate Change and How do They Compare with Natural Influences?
• How are Temperatures on Earth Changing?
• How is Precipitation Changing?
• Has there been a Change in Extreme Events like Heat Waves, Droughts, Floods and Hurricanes?
• Is the Amount of Snow and Ice on the Earth Decreasing?
• Is Sea Level Rising?
• What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era?
• Is the Current Climate Change Unusual Compared to Earlier Changes in Earth's History?
• Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities?
• How Reliable Are the Models Used to Make Projections of Future Climate Change?
• Can Individual Extreme Events be Explained by Greenhouse Warming?
• Can the Warming of the 20th Century be Explained by Natural Variability?
• Are Extreme Events, Like Heat Waves, Droughts or Floods, Expected to Change as the Earth's Climate Changes?
• How Likely are Major or Abrupt Climate Changes, such as Loss of Ice Sheets or Changes in Global Ocean Circulation?
• If Emissions of Greenhouse Gases are Reduced, How Quickly do Their Concentrations in the Atmosphere Decrease?
• Do Projected Changes in Climate Vary from Region to Region?

We'll link to the individual pages once the report is available in html.

RealClimate: Start with our index

Informed, but in need of more detail:

Science: You can't do better than the IPCC reports themselves (AR4 2007, TAR 2001).

History: Spencer Weart's "Discovery of Global Warming" (AIP)

Art: Robert Rohde's "Global Warming Art'

Informed, but seeking serious discussion of common contrarian talking points:

All of the below links have indexed debunks of most of the common points of confusion:

• Coby Beck's How to talk to Global Warming Skeptic
• New Scientist: Climate Change: A guide for the perplexed
• RealClimate: Response to common contrarian arguments
• NERC (UK): Climate change debate summary
• UK Met Office: Climate Change Myths
• Brian Angliss A Thorough Debunking
• John Cross Skeptical Science

Please feel free to suggest other suitable resources, particularly in different languages, and we'll try to keep this list up to date.

Does a global temperature exist? This is the question asked in a recently published article in Journal of Non-Equilibrium Thermodynamics by Christopher Essex, Ross McKitrick, and Bjarne Andresen. The paper argues that the global mean temperature is not physical, and that there may be many other ways of computing a mean which will give different trends.

The common arithmetic mean is just an estimate that provides a measure of the centre value of a batch of measurements (centre of a cloud of data points, and can be written more formally as the integral of x f(x) dx. The whole paper is irrelevant in the context of a climate change because it missed a very central point. CO₂ affects all surface temperatures on Earth, and in order to improve the signal-to-noise ratio, an ordinary arithmetic mean will enhance the common signal in all the measurements and suppress the internal variations which are spatially incoherent (e.g. not caused by CO₂ or other external forcings). Thus the choice may not need a physical justification, but is part of a scientific test which enables us to get a clearer 'yes' or 'no'. One could choose to look at the global mean sea level instead, which does have a physical meaning because it represents an estimate for the volume of the water in the oceans, but the choice is not crucial as long as the indicator used really responds to the conditions under investigation. And the global mean temperature is indeed a function of the temperature over the whole planetary surface.

(more…)

There has been a bit of a flap here at the University of Washington over the state of the snowpack in United States Pacific Northwest region. The Seattle city mayor, Greg Nickels (a well known advocate for city-based CO₂ reduction initiatives) wrote in an Op-Ed piece in the Seattle Times that

The average snowpack in the Cascades has declined 50 percent since 1950 and will be cut in half again in 30 years if we don't start addressing the problems of climate change now. That snow not only provides our drinking water, it powers the hydroelectric dams that keep our lights on.
(more…)

Guest commentary from Juliane Fry, UC Berkeley

The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m², in comparison to the overall average CO₂ forcing of +1.66 W/m².
(more…)

by Rasmus Benestad, with contributions from Caspar & Eric

In a recent article in Climatic Change, D.G. Martinson and W.C. Pitman III discuss a new hypothesis explaining how the climate could change abruptly between ice ages and inter-glacial (warm) periods. They argue that the changes in Earth's orbit around the Sun in isolation is not sufficient to explain the estimated high rate of change, and that there must be an amplifying feedback process kicking in. The necessity for a feedback is not new, as the Swedish Nobel Prize winner (Chemistry), Svante Arrhenius, suggested already in 1896 that CO2 could act as an amplification mechanism. In addition, there is the albedo feedback, where the amount of solar radiation that is reflected back into space, scales with the area of the ice- and snow-cover. And are clouds as well as other aspects playing a role.

(more…)

Michael Mann & Gavin Schmidt

The precise factors underlying the so-called "Little Ice Age" (LIA) have been intensely debated within the scientific community. One key metric in this debate is the spatial pattern of cooling which may provide a 'fingerprint' of the underlying climate change, whether that was externally forced (from solar or volcanic activity) or was part of an intrinsic mode of variability.

Surface temperatures in parts of Europe appear to have have averaged nearly 1°C below the 20th century mean during multidecadal intervals of the late 16th and late 17th century (and with even more extreme coolness for individual years), though most reconstructions indicate less than 0.5°C cooling relative to 20th century mean conditions for the Northern Hemisphere as a whole. There is much less data during these time intervals for the Southern Hemisphere, and that severely limits what conclusions can be drawn there. Just what combination of factors could explain this pattern of observations has remained somewhat enigmatic. A new ingredient in this debate comes with a recent paper in Nature by Lund et al.
(more…)

Sometimes, I encounter arguments suggesting that since we cannot predict the weather beyond a couple of weeks, then it must be impossible to predict the climate in 100 years. Such statements tend to present themselves as a kind of revelation, often in social settings and parties after I have revealed for some of the guests that I'm a climatologist (if I say I work for the Meteorological Institute, I almost always get the question "so, what's the weather going to be like tomorrow?"). Such occasions also tend to be times when I'm not too inclined to indulge in deep scientific or technical explanations. Or when talking to a journalist who wants an easy answer. In those cases I try to provide a short and simple, but convincing, explanation that is easy for most people to understand why climate can be predicted despite the chaotic nature of the weather (a more theoretical discussion is provided in the earlier post Chaos and Climate). One approach is to try to relate the topic to something with which they are familiar, such as to point to empirical observations which most accept (I suppose with hindsight it could be similar to the researchers in the early 20th century trying to convince that nuclear reactions were possible - just look at the Sun, and there is the proof! Or before that, the debate about whether atoms were real or not - just look at the blue sky, and you look at the proof…). I like to emphasised the words 'weather' and 'climate' above, because they mean different things.

(more…)

by Gavin Schmidt and Michael Mann

In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?
(more…)

Guest contribution by Steinn Sigurdsson.

Recently, there have been some suggestions that "global warming" has been observed on Mars (e.g. here). These are based on observations of regional change around the South Polar Cap, but seem to have been extended into a "global" change, and used by some to infer an external common mechanism for global warming on Earth and Mars (e.g. here and here). But this is incorrect reasoning and based on faulty understanding of the data.

(more…)