Climate Change 101 - Part 4 (The greenhouse effect)

Not all of the radiation emitted by the earth’s surface escapes to space. The longwave radiation interacts with gases and water in the atmosphere on its way out. The interactions with gases cause them to vibrate. Here’s how it works …

The important greenhouse gases in the atmosphere are water vapor (H20), methane (CH4), carbon dioxide (CO2), and nitrous oxide (N20). These molecules absorb thermal infrared radiation – which also happens to be the radiation emitted by the earth’s surface. Some wavelengths emitted from the surface escape to space, but others are absorbed by the greenhouse gases. The re-emission of energy from these gases heats up the atmosphere and keeps the earth at a liveable temperature.

Climate Change 101 - Part 3 (Earth's radiative temperature)

Global temperatures are determined by a balance between the incoming and outgoing radation at the top of the atmosphere. On average, 30% of the incoming solar radiation is reflected back to space. The remaining 70% provides heat for the earth’s surface. In turn, the surface heats up and emits longwave radiation in order to keep its temperature at equilibrium. You can calculate the radiative equilibrium temperature by solving for T in this equation:


The left-hand side of the equation is the amount of solar radiation (in watts) that reaches the earth’s surface. S is the solar constant (the amount of radiation emitted by the sun in watts per square meter). α is the albedo of the earth/atmosphere system. The albedo is the percentage of solar radiation reflected. R is the radius of the earth. At any given time, the sun’s radiation hits an approximately disc-shaped portion of the earth. This disc’s area is πR^2.

On the right-hand side of the equation is the amount of radiation emitted by the earth. σ is the Stefan-Boltzmann constant. This constant is used to calculate the rate of energy emission from a blackbody. A blackbody absorbs 100% of the radiation that hits it and it emits radiation at the maximum rate for its given temperature. Despite the name, a blackbody does not have to black. To the first order, we can approximate the earth as a blackbody. At any given time, the entire surface area (4πR^2) of the earth is emitting radiation.


If you want to solve for T yourself: S = 1,367 Wm^-2, α = 0.3, R = 6,378 km, and σ = 5.67*10-8 Wm^-2K^-4. (The unit for temperature is Kelvin. To convert to celcius, subtract 273.)

The radiative equilibrium temperature of the earth is -18°C (0°F). Think about that for a second. According to this equation, the average temperature of the earth is below freezing. However, the observed global mean surface temperature is 15°C (59°F). Obviously, the equation is not representing some essential properties of the earth’s climate.

Climate Change 101 - Part 1 revisited

I reread what I wrote the other day and wanted to say a little more about the contributions of natural forcing to climate change. I don't mean to say that the current changes are ALL due to humans. There is definitely input from natural sources. But the consensus among climate scientists is that these sources are far outweighed by human activities. The 169 authors of the latest IPCC (all of them distinguished scientists in their respective fields) agreed that there is a 90% chance that “the human influence on climate dominates over all other causes of change in global average surface temperature during the past half century.”

I welcome your comments. Let me know what you think! Thanks for reading ...

A large portion of the year-to-year variability in the earth’s climate is due to cycles in El Nino/La Nina. However, while this pattern can explain temperature and precipitation changes on short time scales, it does not account for the strong warming trend seen throughout most of the 20th century. For one thing, the pattern of 20th century warming includes stronger warming over lands than oceans and stronger warming in high latitudes. El Nino is a phenomenom that occurs in the tropical Pacific ocean. So the spatial pattern of El Nino does not match up with the 20th century warming.

There is an 11-year cycle in solar variability, and solar output has increased over the past couple of centuries. However, warming from an increase in solar radiation would be expected to be found in both the troposphere (the lowest part of the atmosphere) and higher up in the stratosphere. But 20th century warming has been greatest in the troposphere and the stratosphere has actually cooled. Increases in solar radiation have contributed somewhat to 20th century warming, but not as much as human activities.

Volcanoes can also contribute to climate change. On short time scales, volcanoes decrease global temperatures. Eras with heavy volcanic activity (such as the Cretaceous) were several degrees warmer than today, but there has not been a significant increase in volcanic eruptions during the 20th century.


Finally, we can use climate models to further assess the importance of human activities versus natural variability. The figure to the left (Figure 9.5 from the IPCC report) shows observed 20th century temperatures (black lines) and modeled temperatures (red and blue lines). In the top graph, the models (red) include both anthropogenic and natural climate forcing. The models agree well with the observed temperature trends, including decreased temperatures following major volcanic eruptions (such as El Chichon and Pinatubo) and a slight decrease in temperatures during the 1950’s and 1960’s. In the bottom graph, the models (blue) use only natural forcings (such as volcanoes, solar variability, and El Nino). These models do not include anthropogenic emissions of greenhouse gases and aerosols. In this case, the models do not show the increase in temperature that we have experienced over the past 50 years. This is further evidence that 20th century warming is mostly caused by human activities.

Source: IPCC FAQ: “Can the warming of the 20th century be explained by natural variability?”
You can download the complete answer to this and other good questions from http://ipcc-wg1.ucar.edu/wg1/FAQ/wg1_faqIndex.html.

Climate Change 101 - Part 2 (Radiation Basics)

The earth’s climate is determined by an energy balance between incoming and outgoing radiation. Everything emits radiation – the sun, the surface of the earth, particles of gas and dust in the atmosphere, clouds, you, and me. The intensity of radiation depends on the temperature of the emitter. Also, the wavelength of the radiation is inversely proportional to the temperature.

Solar vs. Terrestrial Radiation
The sun is a blazing 5500 C and so it emits very intense radiation with short wavelengths. For this reason, solar radiation is sometimes also called shortwave radiation. Most (44%) of the sun’s radiation is in the visible wavelengths (0.4 (violet) to 0.7 (red) micrometers – a micrometer, μm, is one-millionth of a meter). Solar radiation falling outside of this range is not visible to humans, even though it is very intense. The sun’s radiation peaks at about 0.5 μm, which corresponds to the color blue-green. Nearly 37% of the sun’s radiation is between 0.7 and 1.5 μm (infrared wavelengths). Only about 7% of the sun’s radiation is in the ultraviolet range (wavelengths less than 0.4 μm), but this is still enough radiation to cause damage to human cells and cause skin cancer.

The surface of the Earth is on average “only” about 15°C (59°F) and so the radiation it emits has long wavelengths. For this reason, the Earth’s radiation is usually called longwave radiation, or sometimes terrestrial radiation. Terrestrial radiation is almost all in the infrared wavelengths, and it peaks between 5 and 25 μm.

Climate Change 101 - Part 1

Is the climate changing? Is it due to human activity? What should we do about it? These are questions you might have on your mind from hearing about climate change from the media and politicians. While climate change seems like a popular topic right now, most of what you hear about it on a day to day basis is more about the adaptation and mitigation strategies than the science of why and how it is actually happening.

Although the earth’s climate has been constantly changing and evolving since the earth began, the current rate of change is unusual. Global temperatures have increased more rapidly since the Industrial Revolution than during the past thousand years, according to data from tree rings and boreholes. The graph below shows how temperatures have changed since 800 AD according to a handful of data sources. (For more information on this graph, see the IPCC Fourth Assessment Working Group 1 report, Chapter 6 – http://www.ipcc.ch/ipccreports/ar4-wg1.htm). The rapid rate of change, along with its connection to human activities, is why climate change is such an important issue.



Although climate change has been politicized, it’s really a scientific issue. There is a scientific relationship between atmospheric carbon dioxide and global temperatures. Like climate change, this link is nothing new. For example, during the Cretaceous period (145 to 65 million years ago), evidence from fossils suggests that atmospheric carbon dioxide concentrations were much higher than they are today and that global temperatures were several degrees warmer. Almost the entire earth had a tropical climate. The difference between the current warming and the Cretaceous warming is the cause of high CO2. While past causes of high CO2 were natural, today’s primary cause is not. In the past, atmospheric carbon dioxide was high due to increased volcanism and decreased weathering. Today, carbon dioxide is on the rise due to the burning of fossil fuels.

Although how we should respond to climate change is highly debatable, the reasons for the rising temperatures are not. They come down to the physics of radiation and the chemistry of molecular structure. They are topics you probably covered in your middle school science classes. It’s time for a little refresher.