According to internal reports dating back to 1995, scientists working for the Global Climate Coalition, an industry-sponsored group set up to wage a lobbying and public relations war against global warming action, were telling their bosses that human-caused global warming could not be refuted. But, that didn't stop industry lobbyists from waging a cynical campaign to undermine the science and cloud the debate.

Read the full story here.

Americans were outraged a decade ago when cigarette makers made similar claims about the evidence linking smoking and lung cancer. And then we discovered reams of damning research hidden away in tobacco company vaults.

The only real difference between then and now is that global warming stands to threaten more than just people — millions of species face extinction, entire ecosystems altered beyond recognition, the natural world as we know it today irreparably diminished.

Scientists use some technical terms in discussing climate change that can cause confusion. Two that are especially useful to know are "forcing" and "feedback".

You'll hear these terms a lot in discussions of how human activity impacts climate – and especially when the topic turns to the melting Arctic. If you know what they mean, you'll have a much better understanding of the dynamics behind climate change.

"Forcing" and "feedback" both refer to effects on the Earth's climate system. Climate changes when there's a change in climate system's energy balance – the amount of energy absorbed from the Sun, versus the amount of energy radiated back into space from Earth's lower atmosphere and surface. A process that changes this energy balance – and thus the climate – is called a "climate forcing".

Many things can cause climate forcing. Some, like shifts in the Earth's orbit around the Sun or volcanic eruptions, are natural events. Others, like increased concentrations of greenhouse gases in the atmosphere, are caused by human activities. Climate change caused by human activity is called "anthropogenic forcing".

A climate feedback is an indirect (or "secondary") change within the climate system that occurs in response to a forcing. For example, warmer air can hold more water vapor, and water vapor can trap more solar energy in the atmosphere, augmenting the warming effect. This is a positive climate feedback because the secondary effect is in the same direction as the primary effect – both are warming.

Another example of a positive climate feedback occurs with sea ice, which is why you hear the term "feedback" in discussions of Arctic warming. As temperatures rise, sea ice melts and exposes open water. Water is darker than ice so it reflects less sunlight and absorbs more energy. This augments the warming effect, leading to more sea ice melting, and so on in a loop.

Climate feedbacks also can be negative, reducing the initial effect rather than augmenting it. For example, increased water vapor in the atmosphere due to warming can lead to more cloud formation. Thick low clouds can block sunlight from reaching the Earth by reflecting it back into space. This creates a cooling effect – a negative climate feedback.

Positive climate feedbacks are dangerous because they can accelerate climate change towards tipping points, critical thresholds at which a small change qualitatively alters the state of some Earth system. For example, the Greenland Ice Sheet could start a slow but irreversible meltdown if global temperature rises above a tipping point, which scientists warn could be just 2°F above today's temperature.

Tipping points also can trigger climate feedbacks. For example, if the Earth warms enough to melt permafrost (a tipping point for the Arctic region), the melted ground can release greenhouse gases that, in turn, accelerate warming. In fact, studies show that methane emissions are increasing from areas of thawing permafrost in Siberia.

Sound scary? It is. This is why many scientists are pushing hard for legislation to cap greenhouse gas emissions.

You may have heard about the persistent droughts in the western U.S., Australia, and other regions. The Upper Colorado River Basin is experiencing a protracted, multi-year drought that started in 1999. Australia's record drought is threatening the livelihood of traditional farmers and ranchers.

At what point does a passing drought become a permanent shift to desert conditions, and why would such a thing happen?

It can happen because of global warming. Climate change can alter global winds, the strength and location of high and low pressure systems, and other climate factors.

In my post about the California wildfires, I discussed how global warming can lead to more frequent and intense droughts through intensification of the hydrological cycle and earlier snow melts. In this article I'll focus on shifts in global winds.

Global winds shape the Earth's climate, determining – in broad strokes – which areas are tropical, desert, or temperate. Here's a simplified overview of how it works.

The Sun heats the Earth most intensely in the tropical zone around the equator. The heated air rises, cools, and then dumps its moisture as rain. That's why there are rain forests in the tropics.

The now drier air is forced by the continuously rising equatorial air to move towards the temperate latitudes on either side of the equator. At roughly 30° N and S – called the "horse latitudes" – it can move no further due to the Earth’s rotation, and settles to the surface. As the air sinks, it compresses and warms, creating hot, rain-free conditions. This circulation pattern, called a Hadley cell, is why the deserts of the world are located just poleward of the tropics, to the north and south.

Poleward of the desert belt, strong, high-altitude winds known as the jet streams flow from west to east, carrying large storms with them. These mid-latitude, temperate-region storms are an important source of rain and snow, especially during the winter season. Much of the world's population lives in the temperate region. It includes most of the U.S. and southern Canada, most of Europe, East Asia, southern South America, southern Africa, and southern Australia and New Zealand.

But climate regions aren't fixed. Several independent studies have found that global winds are shifting due to global warming, and the shifts are faster than predicted by climate models. Most recently is this new study in Nature Geoscience. The tropical belt has widened by several degrees latitude since 1979. This is consistent with other observations suggesting that the jet streams and storm tracks have moved poleward.

The drought-stricken Upper Colorado River Basin, which includes Lake Powell, is located just poleward of the horse latitudes at around 37° N. This has historically been in the temperate zone, but the desert zone may be gradually encroaching upon it. (Since nothing is simple, there are other factors contributing to this particular drought, as well.) Similarly, water-starved Sydney, Australia at 34° S is just poleward of the southern horse latitude.

What we may be seeing here is not so much drought as desertification – a shift in global climate patterns due to global warming. Areas that used to be in temperate zones may be shifting into desert, while areas that had been arid receive more precipitation.

When we first launched the Climate 411 blog, we posted a lot of useful articles on the basic science of climate change. I often want to link to them in other posts, but I have trouble finding them. If I have trouble finding them, you all certainly must, so here are the links, all in one place.

The Greenhouse Effect Explained
The Water Vapor Fallacy
Picturing a ton of CO2
Picturing 21 Million Barrels of Oil

How We Know Humans Cause Global Warming
1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

Action Needed to Stop Global Warming
1. How Warm is Too Warm?
2. Worldwide Emissions Target
3. U.S. Emissions Target
4. Technologies to Get Us There

A common refrain here on Climate411 is that we need to cut greenhouse gas emissions as soon as possible. One of the reasons we’ve cited numerous times is that, even if we could stabilize greenhouse gas concentrations at today’s levels, some global temperature increase is already locked into the system. This is sometimes called the "warming commitment" or the "warming in the pipeline".

What’s behind this phenomenon? The short answer is: water, water everywhere.

Earth’s vast oceans buffer the atmosphere from large temperature changes because it takes a lot of energy to change water’s temperature. You know this from everyday life: if you put a pot of cold water on a stove and turn on the burner, you can hold your finger in the water for quite a while before the water starts to warm up.

The same principle applies at the global scale. Oceans cover 71 percent of Earth’s surface. It takes an enormous amount of energy to heat that much water, especially since ocean circulation causes warm surface water to sink and cold deep water to return to the surface. As a result, there is a lag time between when greenhouse gases are emitted to the atmosphere and when temperatures start to rise.

How much warming is in the pipeline? It depends on the level at which greenhouse gases are stabilized. For example, the latest IPCC report calculates that if we held greenhouse gas concentrations steady at 2000 levels, average global temperature would go up another degree Fahrenheit. Concentrations have gone up since then, so the warming commitment for today’s greenhouse gas level is slightly higher than one degree.

The implication of this ocean-induced lag time is that we have less time to act than it first appears. The tipping point for losing the Greenland ice sheet may be just 2.3 degrees Fahrenheit above today. If you factor in the warming in the pipeline, about half of that amount is already gone.

In addition to the oceans' role, there’s another factor that makes it so important to act today: the long lifetimes of greenhouse gases. How much of an effect does this have? Well, if we cut global emissions deeply enough, the amount of greenhouse gases in the atmosphere would begin to decline. (A couple decades later, temperatures would decline, too.) Unfortunately, it could take centuries for greenhouse gas concentrations to fall all the way back to today’s levels.

The bottom line is that, thanks to the ocean-induced time lag, for the next few decades we’re committed to additional warming. In addition, because greenhouse gases stay in the atmosphere for so long, every ton of global warming pollution we emit today will affect the climate for decades to come, even if we don’t see the effects immediately.

That's why it’s so important that we start decreasing emissions as soon as possible.

The wildfires in California prompted the largest mass evacuation in California history, destroying half a million acres and over 1500 homes. People are saying that global warming may have played a part in the severity of the blazes. But where is the connection? And how can global warming cause both floods and droughts?

Satellite photo of California wildfires. Source: NASA.

Global warming intensifies the "hydrological cycle" – water evaporates into the air, forms clouds, and then rains back down on the Earth. Higher temperatures cause evaporation to occur more quickly. This can cause very dry conditions on land, even drought. While it's impossible to say for sure that a particular event is caused by global warming, one of the main factors behind the wildfires’ severity was that the 2006-2007 rainy season was the driest on record in Southern California, leaving vegetation extremely dry and flammable.

The greater amount of water vapor rising from the Earth also causes more clouds to form, so when it does rain, the rainfall can be unusually heavy, sometimes enough to cause flooding. Thus some seasons are very wet while others are very dry. In fact, the 2004-2005 rainy season was the second wettest on record for Southern California. During times of extreme wetness, vegetation grows rapidly. Then during times of extreme dryness, all that vegetation becomes fuel for raging wildfires, as happened this year.

Excess fuel comes from another source, as well. Southern California is home to a type of vegetation called chaparral. This shrub land is highly prone to fire, and in fact fire is part of its natural cycle. Fire revitalizes the ecosystem by clearing out small bushes and debris to prevent crowding and restore nutrients to the soil. But people have built homes in this area, so fires are quickly extinguished. When natural wildfires are suppressed, the debris that normally burns away builds up. Eventually a fire breaks out that can't be immediately controlled because there's so much fuel.

Unfortunately, uncontrolled climate change and increasing urban sprawl will likely conspire to create worsening wildfire disasters in California in the future.

And Southern California isn't the only place experiencing worsening wildfires. Wildfires are increasing throughout the Western U.S., both in severity and frequency. So says a 2006 cover story in Science, which collected and analyzed western forest fire data from 1970 to 2003. Since the mid-1980s, the incidence of large wildfires has increased, and the length of the fire season has increased.

Source: Science, August 2006. Parts A and B show that wildfires (Part A, red bars) are more frequent when temperatures (Part A, black line) are high and snow melts early (Part B, black line). Part C shows the length of the fire season (the height of the red and black areas).

Some of this is due to land use and fire suppression, but climate may still be the "primary driver". The high altitudes of the Rockies, where wildfires used to be rare, are now seeing large wildfires with increasing frequency. Warming causes an earlier snowmelt, and "[a]n earlier snowmelt can lead to an earlier, longer dry season, providing greater opportunities for large fires due both to the longer period in which ignitions could potentially occur and to the greater drying of soils and vegetation."

The authors conclude by warning that if western forests burn more frequently, they will release carbon rather than act as a carbon sink, intensifying global warming and leading to even more fires and other harmful consequences.

There is another way that climate change can shift rainfall patterns in a region, and that's through changes in global wind patterns. I'll address this in a future article, when I talk about the drought in the Southeastern U.S.

Last week we got a request to explain the Greenhouse Effect. Here goes.

Source: National Park Service, U.S. Department of the Interior

The Greenhouse Effect is the process by which an atmosphere holds heat around a planet. The story of how this works begins with a discussion of light.

The light that we can see with our eyes is just a small part of the full spectrum of light that occurs in nature. We call this the electromagnetic spectrum because light is a composite of interacting electric and magnetic force fields. The spectrum extends from low energy microwaves and infrared light to the visible part of the spectrum (red, orange, yellow, green, blue, and violet), and then to more energetic forms of light such as ultraviolet and x-rays.

Adapted from NASA diagram. Click to enlarge.

It turns out that all objects with a temperature above absolute zero radiate light. The amount and kind of light that is radiated depends upon the object's temperature. The higher the temperature, the more light is radiated, and the more energetic the light. That's why blacksmiths heating up a horseshoe first see it begin to glow red, and then white as it gets hotter.

The Sun's surface, which has an average temperature of 10,000 degrees Fahrenheit (5810 degrees Kelvin), primarily radiates visible light. That's why we can see sunlight. The surface of the Earth has an average temperature of 60 degrees Fahrenheit (288 degrees Kelvin), so it radiates in the infrared part of the spectrum. We can't see the light the Earth radiates, but we can measure it with modern instrumentation.

The temperature of an object is determined by a balance between incoming and outgoing energy. For the Earth, the incoming energy is the absorbed light from the Sun, and the outgoing energy is the infrared light the Earth radiates out to space. In the absence of an atmosphere with its Greenhouse Effect, that balance would lead to very cold temperatures – well below the freezing point of water.

Fortunately the Earth does have an atmosphere, and that atmosphere contains some molecules with three or more atoms – for example, water (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). These molecules are called "greenhouse gases" because they have a very special property. They do not absorb the visible light from the sun, but they do absorb the infrared light radiated by the Earth's surface.

If greenhouse gases were not in the atmosphere, all the infrared light radiated by the Earth would go back out to space, leaving the Earth too cold for life. But the greenhouse gas molecules absorb the infrared light, and then re-radiate some of it back to the Earth's surface. This makes the surface hotter so it radiates more light, thus establishing an equilibrium at a higher temperature. This process is known as the Greenhouse Effect.

The Greenhouse Effect can be proven in many different ways. For a history of the science, see my previous post, "A 175-year-old Puzzle". More proof can be found by studying Mars and Venus. Mars is very cold, has no appreciable atmosphere, and its temperature can be explained without invoking a Greenhouse Effect. Venus is extremely hot, has a thick atmosphere heavy with carbon dioxide, and only a very strong Greenhouse Effect can account for its high temperatures.

The Greenhouse Effect is a good thing – we wouldn't be here without it. The problem of global warming has arisen because humans are artificially increasing the concentration of greenhouse gases in the atmosphere. This creates what's called an Enhanced Greenhouse Effect.

The atmosphere with its greenhouse gases is the Earth's blanket. We do need some blanket over us or we'd freeze to death, but when the blanket gets too thick, we also have a problem. Excess greenhouse gas emissions and deforestation are starting to make our blanket dangerously too thick. We can't thin it out – greenhouse gases can stay in the atmosphere for hundreds of years. But we can – and must – stop it from getting any thicker.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

Compared to 100 years ago, the temperature of the atmosphere is warmer. No one disputes that. And no one disputes that an extra source of heat must be causing it – that's a basic law of physics. But how do we know that the source of the heat is increasing levels of greenhouse gases from burning fossil fuels and deforestation?

So far in this series I've described how the relationship between atmospheric carbon dioxide (CO₂) and global warming was discovered, how carbon isotopes prove that rising CO₂ concentrations are from the burning of fossil fuels, and how the orbital shifts that cause ice ages cannot explain our recent warming.

That certainly suggests that global warming is caused by increased CO₂ from burning fossil fuels, but how can we be sure?

Besides the greenhouse effect, there are three other heat sources that conceivably could explain global warming:

1. Increased output from the sun.
2. Increased absorption of heat from the sun due to a change in the Earth's reflectivity or "albedo".
3. An internal variation in the climate system that transfers heat to the atmosphere from another part of the Earth.

We know that the increasing greenhouse gases are warming the atmosphere. What about these other possibilities?

It's not the sun. Scientists realized early on that increases in solar output could be the culprit, so we launched satellites to monitor the sun. But the data has shown no appreciable change in solar output for the past three decades:

Source: Foukal et al. 2006. Variations in solar luminosity and their effect on the Earth's climate. Nature 443: 161-166.

It's not Earth's reflectivity. You know how you feel cooler in summer when you wear light-colored shirts? That's because they reflect light rather than absorb heat. The same principle applies to an entire planet. A change in the planet's "color" or brightness can make it hotter or colder by altering its reflectivity. In fact, this may be occurring on Mars, where dust storms appear to blowing light-colored dust from the surface and darkening the planet.

But it's not happening on Earth. Satellite data reveal that the Earth's reflectivity increased (causing cooling instead of warming) from the 1960s to the early 1980s, and has decreased modestly since. The reasons for this are discussed in my earlier post on global dimming. The recent decrease in reflectivity is much too small to account for the current warming.

It's not internal variations. The ocean is the only source of heat capable of warming the atmosphere on decadal time scales. If this were happening, the ocean would be cooling as the heat from the ocean was transferred to the atmosphere. But in fact the ocean has been warming rather than cooling over the past few decades. Whatever is warming the atmosphere is also warming the ocean.

Source: Warming of the world ocean, 1955-2003 [PDF]

The Greenhouse Effect is the only explanation left. As I said at the beginning of this post, it's a basic law of physics that if the globe is warming, the additional heat has to be coming from somewhere. So to end this detective story I'll quote from Sir Arthur Conan Doyle, creator of Sherlock Holmes: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

The only possible explanation for the global warming we're currently experiencing is the Greenhouse Effect, from burning fossil fuels and deforestation.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

In my previous article in this series, I talked about climate change patterns over geologic time. I showed how today's spike in temperature and carbon dioxide (CO₂) concentration breaks patterns and records going back 600,000 years.

This departure can be seen even more clearly when you look at just the last 1000 years (see graph in previous post). So what is this you hear about a Medieval Warming Period?

The Medieval Warming Period refers to a relatively warm period lasting from about the 10^th to the 14^th century. Some argue that since the Medieval Warming Period was a natural event, the current warming also must be due to natural processes.

This argument is wrong on two counts – one factual and one logical. I'll start with the logical error.

Let's say that the premise is true – that there was a global warming period in Medieval times. (It's not true, but for the sake of argument, let's say it is.) It does not then follow that the current warming period has the same cause! If you and I both have purple shoes, it does not then follow that we got them in the same place. This logical fallacy, commonly used by climate change skeptics, is called Ignoratio Elenchi – Latin for "irrelevant conclusion".

So the logic makes no sense to begin with, but in this case even the premise is wrong! There may have been a warm period during Medieval times, but current studies show that (a) temperatures were not as warm as today, and (b) it wasn't global.

There's been a lot of argument and misinformation on this topic. For a great summary of the history, see "Myth vs. Fact Regarding the "Hockey Stick"" on the RealClimate Web site.

The graph below is from a sidebar in the IPCC assessment called "Hemispheric Temperatures in the 'Medieval Warm Period' [PDF - 7.9MB]" (page 468). As you can see, when you look at temperature estimates from around the world – not just Western Europe – there's no clear spike in temperature until our own time.

If you don't trust the temperature proxies, here's proof that is perhaps even more telling. Andean glaciers that have been intact for more than 5,000 years are now rapidly melting (see original study, news coverage). If the Medieval Warm Period was truly global, these glaciers would not have survived.

So there's one more climate change myth debunked. Hopefully this article will help you to explain the "Medieval Warming Period" to others. Any comments? Questions? Post them here!

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

An argument I hear frequently from climate change skeptics goes like this: "Climate has undergone warming and cooling cycles for millennia. This is no different. It's just another naturally occurring warm cycle."

It's true that climate has undergone warming and cooling cycles for millennia, but it's not true that this is just another naturally occurring warm cycle. Here's why.

Scientists have made a study of the history of climate change (the field is called "paleoclimatology"), so we know quite a bit about past shifts in global temperature and what caused them. Carbon concentrations and temperatures can be tracked back hundreds of thousands of years by looking at ice cores, and millions of years using other proxies.

When using ice cores, CO₂ concentrations are measured from air bubbles in the ice, and temperature is estimated from hydrogen isotopes in the water. (See my previous post in this series for an explanation of isotopes.)

The graph below, which is based on ice core data, shows CO₂ concentrations and temperature from 400,000 years ago to the present. You can't see the 1°F to 3°F differences in global temperature that we are concerned with today because the temperature estimates shown in this graph are local for Antarctica, which has experienced less warming than the global average. Still, it shows that temperature has varied greatly over time, and this variation is strongly correlated with CO₂ concentrations.

Data Sources for CO₂: Vostok ice core, Law Dome ice core, and Mauna Loa air samples.

Data Source for Temperature: Vostok ice core.

The major dips in the graph are ice ages, caused by shifts in the Earth's orbit around the sun. The warming from the change in how sunlight hits the Earth is amplified by an increase in CO₂ concentration. The sequence goes like this:

1. Orbital changes trigger an initial, relatively small temperature change on Earth.
2. Melting permafrost and other changes caused by the warming release CO₂ into the atmosphere.
3. Since CO₂ is a greenhouse gas, increased CO₂ concentrations cause more global warming.

You also can see smaller ups and downs before modern times. These were caused primarily by variations in solar output and volcanic activity.

Now look at CO₂ and temperature in the recent past – the far right of the graph. See how CO₂ is rocketing up, and higher today than it's been in the last 400,000 years? Actually, CO₂ concentrations are higher today than they've been in the last 650,000 years – this graph just doesn't go back that far.

Let's focus in on more recent history so we can get a better look. The graph below zooms in on just the last 1000 years, using temperatures for the whole Northern Hemisphere, not just Antarctica.

Data Sources for CO₂: Law Dome ice core and Mauna Loa air samples.

Data Source for Temperature: NOAA.

The natural phenomena that caused climate change in the past cannot account for what we see today. We're already in a warm period orbitally, as you can see from the first graph, and now suddenly CO₂ concentrations and temperatures are skyrocketing.

This break in the pattern – even if you had no other information – strongly suggests something different is happening today. And as the other posts in this series show, something different definitely is happening. Today's abnormally high temperatures and CO₂ concentrations come from the burning of fossil fuels. As you'll see in the last article in this series, nothing else can account for it.

Next up: the Medieval Warm Period. Stay tuned!