Radiative Forcing

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There have been a few times we’ve used the term “forcing” when talking about greenhouse gases and climate change. And the term comes up repeatedly when you read a news article about climate change, watch a TV spot, or listen to a podcast. But what does the term actually mean?

Radiative forcing is essentially a way to measure how different factors affect the Earth's energy balance. Remember when we talked about the energy budget? The sun shines on Earth, the planet absorbs some of this shortwave energy and reflects the rest back into space. At the same time, Earth is constantly emitting energy as longwave radiation. The balance between the energy Earth absorbs and emits to space determines our climate. Any factor that disrupts this balance—either by trapping more heat or allowing more heat to escape—is what we call a "radiative forcing."

Here’s a way to think about it: picture a mug of hot cocoa sitting on a warming plate. At some point, the cocoa will reach a steady temperature because it’s being heated from below but also losing heat to the air around it. Now, if we wrap the mug in insulation, what happens? The cocoa heats up beyond that steady point because it’s no longer losing as much heat to the room. That insulation is acting like a "forcing" in our cocoa system, changing the balance of energy. This is exactly what radiative forcing does in Earth's climate system!

Two mugs of hot cocoa on warming plagues. The mug on the right is warmer even with the same energy input (same setting on the warming plate) because it is insulated (i.e., positive forcing) and retains heat more effectively. Greenhouse gases in the atmosphere act similarly.

Credit: DALL-E

Estimated radiative forcing on the climate system from a variety of human activities and natural sources since 1750.

Credit: Estimated radiative forcing, Environmental Protection Agency (EPA) (Public Domain) 

Take a look at this figure, which captures our current understanding of radiative forcing caused by different atmospheric components. The length of each bar represents how much each agent is affecting the climate—essentially, how much warming or cooling it’s contributing. Red bars indicate warming, while blue bars represent cooling. You’ll notice the horizontal “barbells” attached to each bar—these are error bars, showing how uncertain we are about the exact effect of each factor. Longer error bars mean greater uncertainty, so keep an eye on them as you interpret the data.

The units here are watts per square meter (W/m²), which essentially tells us how much energy each agent adds or removes from the Earth system. You can think of it as a way of quantifying “how much warming or cooling” each factor contributes to the planet’s energy budget.

The figure breaks down these factors into two major categories: those caused by human activities (anthropogenic) and those that occur naturally. The top section of the chart lists human-driven changes—things like greenhouse gas emissions. You’ll notice that the 'big four' greenhouse gases we’ve talked about (carbon dioxide, methane, halogenated gases, and nitrous oxide) all show significant warming contributions. Interestingly, halogenated gases (HGs) have a slight cooling effect in addition to warming because some of these gases destroy ozone in the stratosphere, reducing the amount of shortwave radiation the Earth absorbs. This dual role makes them a bit more complicated than other greenhouse gases.

Next, let’s touch on the "short-lived gases" rows. These are gases like carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), and nitrogen oxides (NOx). We haven’t spent much time on these yet, but they’re worth mentioning. These gases don’t last long in the atmosphere—anywhere from a few hours to a few days—but they play an important role in the climate system. Even though they aren’t major greenhouse gases themselves, they drive chemical reactions that influence the concentrations of other climate forcers like ozone and methane. For example, NOx reacts with sunlight to produce ground-level ozone (a potent greenhouse gas and pollutant), while CO and NMVOCs help prolong the lifespan of methane in the atmosphere. These gases come mostly from combustion processes, like vehicle emissions and industrial activities. While they don’t directly cause warming, their role in enhancing other pollutants makes them key players in short-term climate and air quality—think of them as carbon dioxide’s morning coffee, giving it an extra kick.

Right below, we have aerosols, split into warming (black carbon) and cooling (sulfate aerosols) effects. The red portion of the bar represents black carbon (soot), which absorbs sunlight and warms the atmosphere, while the blue portion represents sulfate aerosols, which scatter sunlight and cool the Earth’s surface. The two effects nearly cancel each other out, but as you can see from the long error bar, there’s a lot of uncertainty surrounding the net impact. Directly beneath this is "Changes in clouds due to aerosols," which refers to the secondary effects of aerosols acting as cloud condensation nuclei. More aerosols mean more cloud droplets, which makes clouds more reflective, bouncing more sunlight back into space. So, in this case, “dirtier” air can lead to a cooling effect by brightening clouds. We’ll explore this concept later when we talk about "cloud seeding" as a proposed climate solution.

The last row under human activities is about land-use change, which we’ll cover in more detail soon. Finally, we have the sole natural source of variability—changes in solar energy due to long-term trends in sunspots. As we discussed in the last lesson, this effect is relatively small compared to the human-driven factors shown above.

Understanding these factors and their impacts on radiative forcing helps us comprehend how they shift the Earth's energy budget, leading to changes in climate. As we progress through this course, we'll explore how these changes, whether warming or cooling, have shaped the Earth's climate over the past century and what that means for our future.