Radiative forcing boils down to the difference between energy coming from the sun, versus how much energy leaves the Earth and escapes into space. Higher radiative forcing means that more of the sun’s energy is getting stuck in our atmosphere. Greenhouse gases (GHGs) prevent heat from escaping back into space, so as we have increased GHGs by burning fossil fuels, we have also increased radiative forcing.

Imagine a pot of boiling water. If you keep the lid on, it traps the heat, and the water boils faster. If you leave the lid off, some of that heat escapes into your kitchen, and the water doesn’t warm as quickly. By increasing the GHG concentration in our atmosphere, we have put the lid on our pot.

How do we measure it?

Each GHG absorbs and releases heat differently. Basically, we measure the concentration of each gas in the atmosphere, and then we calculate the warming influence of that gas. Once we add them all together, we have our radiative forcing.

CO2 concentration ties into radiative forcing in that it is a potent GHG, but how do we know it is increasing and that it comes from fossil fuels?

How do we measure CO2 concentration?

There is a volcano in the middle of the Pacific Ocean called Mauna Loa, where CO2 in the atmosphere is measured every day. Mauna Loa is so remote and tall that there isn’t contamination from direct CO2 sources, so it is a great place to measure CO2 in real-time in the atmosphere. You can actually see the Northern Hemisphere “breathing” in the record—CO2 dips during the summer when plants are actively growing and rises in the winter when they aren’t. This record is called the Keeling Curve, and it has been measured since 1958.

What is being measured is not technically a concentration, because measuring concentration would depend on external factors like pressure and temperature, and would make the measurement inconsistent. Mauna Loa is actually counting the number of CO2 molecules that are found in a million molecules of dry air (so after any water vapor is removed). It is generally called a concentration because that is an easier concept to understand than what it is really called: a mole fraction.

By the time a mass of air reaches Mauna Loa, it is well mixed and representative of the Northern Hemisphere as a whole, but why is this station located in the Northern Hemisphere? Most of the activities that affect CO2 levels are located in the Northern Hemisphere: manufacturing, automobiles, coal power plants, etc. This makes it a good representation of how much CO2 is increasing overall, but you will also see a similar trend in the growth of CO2 at other measurement locations around the world because the rate at which CO2 increases is fairly steady globally, it is well mixed in the atmosphere, and it stays there for about 100 years.

Note: The Mauna Loa volcano erupted in November 2022, and measurements were disrupted until July 2023. In between, the official Keeling Curve measurements were replaced by a station on the Big Island of Hawai’i called Maunakea.

How do we know this is high?

We actually have a record of atmospheric CO2 concentration going back 800,000 years, thanks to ice cores that have been retrieved from the ice sheets on Greenland and Antarctica.

How do we know if CO2 comes from fossil fuels?

This all has to do with the three different types (isotopes) of carbon. Since we know how much of each type of carbon should be in the atmosphere, any changes in those numbers will indicate a change in the source of the carbon.

Think of it like this: Your friend Bob gives you one apple a day, and your friend Sue gives you one pear every day. If you don’t eat anything, you know you will have 50% apples and 50% pears in your fridge. If you end up with more than 50% of either fruit, you know who it came from.

Proof point #1: When we burn fossil fuels, we are increasing the concentration of C12 in the atmosphere

Proof point #2: When we burn fossil fuels, we decrease the concentration of C14 in the atmosphere.

This has to do with how quickly different species can go extinct before it negatively impacts ecosystem function. Extinctions are measured in “extinctions per million species-years” (E/MSY), which really means for every one million species there are on earth, one species will go extinct every year.

Under “normal” extinction rates (how many species went extinct before humans started messing with the planet), up to 2 species across all of the animal groups can be expected to go extinct every million years (and that is a generous estimate; others put the number between 0.1 and 1). So, how many extinctions are we experiencing now? More than 100 E/MSY. That means that humans are causing species to go extinct 50 to 1000 times faster than they should, and we’ve lost more than 10% of our planet's plant and animal diversity over the last 150 years because of it.

Here, they are looking at net primary productivity (NPP) and specifically how much NPP humans have taken away from the biosphere. NPP is the amount of biomass (carbon) that goes into the biosphere minus the energy it uses to consume that carbon, so you can think of it as a measure of carbon sequestration.

A natural forest has a higher NPP than cropland, so it sequesters more carbon. As people have been converting forests and grasslands over to agriculture, we have reduced the planet's NPP, and that means we have reduced the amount of carbon that can be drawn out of the atmosphere. The biosphere should have an NPP of 71.4 gigatonnes (Gt) of carbon/year (based on what it was before us and how much CO2 is in the atmosphere now), but it is currently at 65.8 Gt C/year.

How do we measure it?

First, there are two terms to know:

NPP is the difference between photosynthesis and respiration. To get this value on a global scale, we can divide the planet into grids and look at the ecosystems contained within each grid. Based on the type and coverage of vegetation there, we can calculate the NPP for each grid, and then we add them all together. Here is a video showing how it’s done if you’re curious.

This boundary looks at how much natural forests have been lost since people began to change the world around them. Since people started settling down, we have been cutting down forests and converting that land into agriculture, buildings, roads, parking lots, shopping malls, really the list is almost endless. Forest fires are another cause of rapid land system change.

The reason they are looking at old, diverse, natural forests and not just tree cover is because these ecosystems are much more biologically active and sequester more carbon than most reforestation projects (i.e. planting new trees to replace what has been cut town). More than half of all reforestation is in the form of intensively managed monoculture - meaning one type of tree planted in the same way we grow any crop. These tree plantations trap about 40 times less CO2 than a natural forest would have if left alone and can even end up being a source of emissions.

How was blue water measured?

Streamflow, literally the flow of water through streams and rivers, is used to represent blue water in this new assessment. Streamflow is a good indicator of human impacts on blue water because it is impacted by factors like water withdrawals, groundwater depletion, and human-caused climate change. While it is possible to directly measure streamflow, those measurements aren’t widespread enough to make a proper assessment, so hydrological models were used.

Why is this important?

Life on Earth cannot exist without water. The ecosystems on our planet, both land-based and aquatic, have developed around specific amounts of water, and the timing in which they receive that water. If the amount and timing of water changes, it can have severe effects on the health and biodiversity of an ecosystem.

How was green water measured?

This boundary looks at all of the ice-free land areas on Earth and how much water is available to plants in the soil (specifically the soil depth where the roots of the plants are located).

Soil moisture was selected as the variable to measure because it was found to best represent human disturbances on green water, and could account for regional shifts. Since we don’t have widespread direct measurements of root-zone soil moisture, the values here are also estimated by hydrological models that analyze records like precipitation, evaporation, surface soil moisture, temperature, etc.

Why is green water important?

The management of green water is very important to land-based ecosystems as well as global water and climate cycles. Let’s look at the Amazon rainforest as an example. The soils of the Amazon rainforest are drying out, which means the trees are stressed and dying. A rainforest is called a rainforest because the trees there literally make their own rain, so when there are less trees, the forest loses this rain generation ability and becomes even dryer. Also, since the trees are struggling, they are absorbing less CO2, and parts of the rainforest are even becoming a source of emissions.

There are certain elements that are fundamental to life on Earth. Carbon is a big one, but we’ve already covered that. Phosphorous (P) and nitrogen (N) are two more essential elements because these are nutrients that plants need to grow.

P and N are naturally available, but we also add a lot of them to the environment through a range of activities. One of the major sources is commercial fertilizer for agriculture, but they also come from livestock production, cleaning products, burning fossil fuels, and wastewater, among others.

What can go wrong? Here are a couple of examples:

Novel entities mean anything that has been introduced into Earth’s systems that does not come from nature. So this can be microplastics, antibiotics, nuclear waste, pesticides, and much more.

It has been difficult to quantify the amount of novel entities going into the environment because we are producing them faster than we can actually measure them—especially synthetic chemicals where there are hundreds of thousands already out there. In fact, 80% of the chemicals in use over the past 10 years, according to those registered under the EU REACH regulation (a small sample of everything out there), haven’t undergone a safety assessment.

One of the largest contributors to novel entities in the environment is the plastics industry. The total mass of plastic in our world is more than double that of all living mammals on the planet. Only about 9% of plastic produced actually gets recycled to make new plastic.

Ocean acidification doesn’t mean the oceans are becoming acidic, but rather that they are becoming less basic. Try to remember back to the pH scale you learned about in science class, one end has a low pH (acids), and the other has a high pH (bases). Ocean water is naturally basic. When CO2 dissolves in the ocean, it combines with becomes carbonic acid, and this decreases the pH of the water, making it less basic.

Aragonite is a mineral (calcium carbonate) that ocean creatures like snails and mollusks use to produce their shells. If there isn’t enough aragonite dissolved in the water, these creatures will not be able to build the shells and skeletons that they need to survive. Ocean acidification decreases the amount of aragonite that is available to ocean life, so measuring aragonite saturation is a good indicator for ocean acidification.

Why is this important?

Ocean acidification is threatening the balance of our oceans. It is changing marine food chains, which will have an effect on the food supply for humans, but there are other less obvious effects to consider. Coral reefs, for example, are important for protecting coastlines from waves and storms. Corals, are made of calcium carbonate, so acidification slows down the growth of corals and at a certain point, they will start to erode away faster than they can grow.

Let’s start off by defining the terms here.

If we bring it all together, the mean annual interhemispheric difference of atmospheric aerosol loading is the difference in the amount of aerosols in the atmosphere between the hemispheres, in this case, the Northern and Southern Hemispheres.

How is this measured?

The variable being measured here is aerosol optical depth (AOD). Aerosols can block sunlight from getting to the surface of the earth, and AOD shows how much the sunlight getting through has been reduced.

Why is it important?

Aerosols can have a cooling or warming effect, depending on the type, size, and location of the aerosol. They are also very important for cloud formation and precipitation. Water needs something to cling to in order to form clouds and eventually raindrops, so it uses the aerosols in the air. Changes in the concentration of aerosols between the Northern and Southern hemispheres can have a lot of consequences on the health of our planet and everything that lives on it.

There is something called the “Intertropical Convergence Zone” (ITCZ), which is a region in the tropics where the winds of the Northern and Southern Hemispheres come together. When these winds meet, the air is forced up, creating thunderstorm clouds. The ITCZ moves around seasonally, following the movement of the sun, and this creates the rainy and dry seasons in tropical areas. If there are many more aerosols in the Northern Hemisphere, this will cause a cooling effect and push the ITCZ further south, leading to droughts in the areas where those clouds should have been and pushing other weather patterns (like El Nino) out of whack.

The mechanisms behind the influence of aerosols on the ITCZ is still being studied, so we will hopefully learn more about how and why these effects occur in the near future.

Ozone is a molecule made up of three oxygen atoms. We have a concentrated layer of ozone in our stratosphere that acts like a shield against solar radiation. In particular, the ozone layer filters out and reflects the most damaging radiation (the kind that causes cancer). The ozone layer is measured in Dobson units (DU), which represents the thickness of the layer in a vertical slice of the atmosphere. A higher DU means a thicker ozone layer.

In the 1980s, we discovered that our behavior (like using hairspray and refrigerants) had created a hole in the ozone layer over Antarctica. In preindustrial times, the ozone layer was 290 DU, and this hole we discovered was down to 109 DU in 1987. The truly amazing thing here is that the world came together to phase out the chemical compounds that were damaging the ozone layer by enacting The Montreal Protocol. It took some time, but now the ozone layer is regenerating itself.

This is a great example of how people can come together to solve a large environmental challenge in the face of a looming disaster. We did it once, and we can do it again with climate change. For the ozone layer, we needed to ban the chemicals that were causing the damage, and we did. To save our climate, we know what needs to be done: stop burning fossil fuels and stop chopping down natural forests.

Note: Refrigerant compounds (like the ones that were creating the ozone hole) also play a large role in global warming and each molecule can absorb thousands times more heat than CO2. Learn more in our research into cold storage.