3 December 2010

Ask a Climate Scientist – Answers from AGU Researchers

Posted by koneil

IMAGE CREDIT: Andrew Gavin Marshall, Global Research.

Australian journalist Amber Jamieson thought AGU’s Climate Q&A Service, which re-launched in October to provide journalists quick responses from scientists to questions about climate change, provided a great, but under-used opportunity. So she solicited questions from readers of her blog, Rooted, and sent them to AGU’s climate scientists for answers. She posted the responses on her site.

This is the first of a series of posts with those questions and answers.

Q1. I would like to learn more about “forcings.” People often say “but the climate has changed before, why do scientists conclude that on this occasion it is caused by human activity?”

A1. “Forcings” are just another term for drivers of climate change and can arise from all sorts of different sources. One forcing would be the changes in the sun’s brightness for instance as a function of the 11-year solar cycle. Another natural forcing arises from the impact of large volcanic eruptions, which can put significant amounts of reflective ‘aerosols’ (or particulates) in the stratosphere. Going further back into the past, the closing of isthmus of Panama about 4 million years ago (because of plate tectonics) was a forcing that altered climate and ocean circulation in the North Atlantic.

Over the twentieth century though, it is human activity that has provided most of the forcings. That activity has changed the amount of carbon dioxide and other greenhouse gases in the air, altered the concentration of aerosols, and chopped down forests that change how sunlight is reflected from the surface. Each of these factors has been a forcing, and we are pretty sure that the net effect of all of these changes has been to drive the climate towards warming. The natural forcings (from the sun and volcanic activity) are mostly neutral over this time period, perhaps even pushing towards cooling in the most recent decades.

The recent changes in CO2 or methane have been caused by human activity – mainly the burning of fossil fuels, deforestation for CO2 and agriculture, landfills and mining for CH4 – and so while these constituents have varied in the past (over the ice age cycles for instance), the reasons they varied then are not why they are changing today.

Gavin Schmidt, NASA/Goddard Institute for Space Studies

Reviewed by:

Robert Jacob, Argonne National Laboratory

Q2. Carbon dioxide has been given to explain the Little Ice Age and Mediaeval Warming. What is the evidence that CO2, or some other human-sourced greenhouse gas, has caused the increase in temperature since 1860?  Is it just a positive correlation between CO2 and temperatures, or is there some other, more fundamental reason, keeping in mind, as I understand the argument, that the ability of CO2 to absorb long-range radiation increases only in  a logarithmic fashion and that the amount of long-wave radiation absorbed by CO2 is miniscule compared to other greenhouses gases, such as water vapour, of which the presence of the latter doesn’t seem to me to  be due directly to human activity?

A2. It is not true that CO2 explains the Medieval Warming or the Little Ice Age. Natural causes of climate change were dominant until about 1950, and human causes have dominated after that. The evidence is in climate model simulations forced by these different factors.  Anthropogenic greenhouse gases (GHGs) like CO2 trap enough extra heat to explain about half of the observed warming since 1750. Models show that the other half of the warming is due to self-enhancing feedbacks in the climate system, including the increase in water vapor (a strong GHG) due to the anthropogenic CO2.

Charlie Zender, Professor of Earth System Science, University of California, Irvine

-AND-

Alan Robock, Professor II, Director, Meteorology Undergraduate Program, Associate Director, Center for Environmental Prediction, Department of Environmental Sciences, Rutgers University

Reviewed by:

Andreas Schmittner, Assistant Professor, College of Oceanic and Atmospheric Sciences, Oregon State University

Q3. How can carbon dioxide, which forms such a tiny portion of the atmosphere in parts per million, cause such a great flux in temperature?  It doesn’t sound plausible.

A3. A simple answer is to compare CO2 with a pinch of salt in an otherwise tasteless, flat soup. A little salt can make a big gastronomic difference.

A more scientific answer would be: In the atmosphere there is water vapor. Water stops IR (infrared) radiation more effectively than CO2, but CO2 increases temp which increases evaporation, that is water content in the atmosphere, which increases temperature, which increases evaporation, and so on. Also, Oxygen and Nitrogen, the main components of the atmosphere are essentially transparent to the IR radiation, while CO2, water, methane, nitrous oxide, ozone and CFCs are opaque to it at certain wavelengths. The tri-atomic molecules of CO2, water and ozone are typically dipolar and thus  ‘resonate’ (oscillate in unison) with the outgoing electromagnetic IR radiation, which gets them agitated (hot) and thus capable of sending part of that agitation back to the earth’s surface in the form of extra heat. The diatomic molecules of oxygen and nitrogen are not dipolar and thus do not resonate with either the visible or the IR radiation.

The other analogy is the microwave oven. The water in the food (even if there is very little of it) resonates with the microwaves (close in wavelength to the earth’s IR radiation) and cause the food to warm from the inside.  Similarly, greenhouse gases are heated up by the IR radiation and heat the atmosphere from the inside.

J. A. Rial, Professor, Department of Geological Sciences, University of North Carolina at Chapel Hill

Reviewed by:

Andreas Schmittner, Assistant Professor, College of Oceanic and Atmospheric Sciences, Oregon State University

Q4. Why do so many people (politicians, media etc) say that anthropogenic climate change is a ‘fact’ when (1) the IPCC says that it is “highly likely” that climate change is due to anthropogenic factors, and (2) that it is impossible to prove the null hypothesis, but you can only demonstrate high correlation? Doesn’t the continued proclamation of “fact” damage scientific reputation and the message that everyone is trying to get out?

A4. What is a fact?  To a scientist, there are only prevalent theories, and because it is possible that new evidence or understanding, however unlikely, could appear to disprove the current theory, it is hard for us to say something is a fact.

But in my opinion, if something is very likely, it would improve communication with the public to call things facts, and it is good that politicians and media say that.  Is it a fact that Earth is round?  Is it a fact that gravity pull things downward?  Yes. Yes.  Is it a fact that Earth is warming?  Yes.  Is it a fact that the greenhouse gases humans add to the atmosphere cause warming?  Yes.  Is it a fact that natural causes of climate change (volcanic eruptions and solar variations) cannot explain the warming of the past 50 years?  Yes.  Is it a fact that the warming caused by the greenhouse gases, particularly carbon dioxide, but also methane, nitrous oxide, and Freon, can explain the warming?  Yes.  Is it a fact that natural variability of the climate system (chaos) cannot explain the warming?  Yes.  Is it a fact that there is no explanation other than the effects of humans that can explain the warming?  Yes.  So I think this is the way, using the word “fact,” that we should explain global warming.

Alan Robock, Professor II  Director, Meteorology Undergraduate Program, Associate Director, Center for Environmental Prediction, Department of Environmental Sciences, Rutgers University

Reviewed by:

Andreas Schmittner, Assistant Professor, College of Oceanic and Atmospheric Sciences, Oregon State University

Q5. These are actually not my questions– they are raised by notorious climate sceptic Roy Spencer, who says the questions are never answered by scientists. So let’s answer them” The possibility that small changes in ocean circulation have caused clouds to let in more sunlight is just one of many alternative explanations which are being ignored” Has it been ignored? Does it make sense?”   Not only have natural, internal climate cycles been ignored as a potential explanation, some researchers have done their best to revise climate history to do away with events such as the Medieval Warm Period (MWP) and Little Ice Age. This is how the ‘hockey stick’ controversy got started.” Have climate scientists ignored ‘natural climate cycles?” like the MWP and Little Ice Age? “If you can get rid of all evidence for natural climate change in Earth’s history, you can make it look like no climate changes happened until humans (and cows) came on the scene.” Is this true?

A5. Brief responses regarding these two related points. I can dig up references if it would be helpful- IPCC FAQ is pretty good on these questions though.

“Not only have natural, internal climate cycles been ignored as a potential explanation, some researchers have done their best to revise climate history to do away with events such as the Medieval Warm Period and Little Ice Age. This is how the ‘hockey stick’ controversy got started.”

Have climate scientists ignored ‘natural climate cycles?” like the MWP and Little Ice Age?

I’m not aware of any scientists trying to ignore these natural climate events. On the contrary, they are extremely valuable to climate scientists as they test our ability to understand how the climate system works. They are studied intensely — 3206 papers on the Web of Science published in the last 10 years on the topic Little Ice Age Climate and 472 papers on the topic ‘medieval warm period’.

“If you can get rid of all evidence for natural climate change in Earth’s history, you can make it look like no climate changes happened until humans (and cows) came on the scene.” Is this true?

Earth exhibits natural climate variability, there is no question about that. There’s no reason that human-induced change cannot be occurring in addition to this natural variability, and this is what all of the available evidence suggests is happening. If models can reproduce natural variability by including all known natural forcing (this is broadly true but details are still being worked out), but cannot reproduce post-industrial variability without including anthropogenic forcing (this is definitely true), this is strong support for human-induced climate change.

Zanna Chase, Senior Lecturer, Institute for Marine and Antarctic Studies, University of Tasmania

Reviewed by:

E. Christa Farmer, Assistant Professor of Geology, Hofstra University