It’s been widely accepted for some time now that the earth is getting warmer, that we humans have quite a lot to do with this, and that this is very bad news for us as a species. Headlines like “NASA predicts the end of Western civilization” don’t exactly reassure anyone on that front, either, since apocalyptic futures are much more fun in fiction than in real life. And so for a lot of people, this is the depth of their understanding on climate change: they’ll read an article summarizing a climate study that says our global doom is imminent, feel a brief surge of panic, and then deliberately put it aside so that creeping existential dread doesn’t keep them from going about their day. It doesn’t help that a lot of the jargon used may be unfamiliar, or that even familiar words like “model” or “scenario” may mean very specific things that aren’t unpacked for the reader’s understanding. So let’s unpack them here. I’ll give a brief review of what we talk about when we talk about climate change, a discussion of some of the terminology, and talk a little about why it’s so complicated to begin with and what it all means.

So, climate change: this term could refer to any significant change in weather patterns over a long-term period. (“Long-term” could mean from decades up to millennia; this is why you should roll your eyes when cranky types cite a single winter storm as evidence that the planet can’t possibly be getting warmer.) I’ll be using “climate change” rather than “global warming” throughout this piece, as the effects on climate I’ll be discussing extend beyond just higher temperatures. But yes, the climate change we are referring to here is that average temperatures around the globe have been on the rise since the Industrial Revolution.

Why is this happening? “Because pollution” is the common understanding, but let’s delve into that a little bit more. Imagine the earth is all snuggled up in a warm, fluffy blanket:

that’s our atmosphere. Like a fabric made of a blend of different fibers, the atmosphere is made up of many different gases, each of which has different properties. And some of these gases, it turns out, are really good at absorbing and retaining heat – which you’d want in a blanket, of course. We call those greenhouse gases (GHGs for short): they include carbon dioxide, methane, nitrous oxide, and plain old water vapor, among others. They’re pretty good, in moderation! They maintain a relatively consistent temperature appropriate for life, keeping our planet from being an icy, lifeless rock; that’s pretty good.

How do GHGs do this? Well, the sun is constantly radiating energy at our planet. A lot of that light and heat would be lost uselessly to space (either never making it to Earth in the first place, or reflecting off Earth’s surface and back out into the vast cosmic deep) if it weren’t for our atmosphere’s ability to absorb and retain this radiation, keeping temperatures relatively consistent under the blanket. This, in a very simplified nutshell, is the greenhouse effect. It’s not too hard to understand, therefore, that when the concentration of GHGs in the atmosphere starts to increase, heat retention increases too. We get warmer and warmer until the planet starts feeling like a big sweaty mess.

The question before us, then, is how big and how sweaty of a mess are we going to become? While it’s easy to say, “Oh no, we’re increasing greenhouse gases, things will get warmer and that’s bad,” it’s harder to quantify “Exactly how warm, and when? And bad in what ways, and exactly how bad, for that matter?”

This is where we have to start to predict the future.

Let’s start with just one question: How much warmer is it going to get? This is nowhere near as straightforward as it may sound. We’ll leave aside for now any questions of whether the earth is simultaneously entering a natural cyclical period of increasing warmth that’s non-manmade. Instead, we’ll focus exclusively on how humanity is going to affect the atmosphere in the future, and consequently affect the future climate.

When you think about it, our impact on global climate depends on a number of things we don’t yet know for sure. How many people will there be on Earth at different dates in the future, and where will they live? How will they use energy – continuing to rely on fossil fuels, which put out lots of GHGs, or incorporating more renewable sources? If they do increase use of renewables, what does that transition look like? How will different areas of the world develop economically – rapidly industrializing, as much of the West did, or in another way the planet hasn’t yet seen? How will land be used throughout the world – for industry, for agriculture? (Don’t assume agriculture’s all that benign; methane is significantly more potent per unit as a GHG than carbon dioxide, and cow burps, for one thing, are a massive source of methane.)

We can’t just guess at the answers to these questions if we want to make realistic predictions about future climate change, because these quantifiable factors are what will shape it. Therefore, to build these predictions on solid ground, the scientific community uses emissions scenarios as consistent starting points. These scenarios can be thought of as storytelling about what human civilization on Earth will be like by some future date. Each describes a different combination of potential patterns of population growth, resource use, development and governance, ultimately arriving at each scenario’s likely output of greenhouse gases over time based on these factors. Typically, scenarios project from the present through 2100.

Similar scenarios are clustered into 4 different families, and climate scientists choose from among them based on the type of future they are predicting for. (Often, multiple scenarios are run through a model, to generate a range of different predictions based on a range of different possible futures.) Here are summaries of each scenario family from the Intergovernmental Panel on Climate Change:

• The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B).
• The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing global population. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines.
• The B1 storyline and scenario family describes a convergent world with the same global population that peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.
• The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with continuously increasing global population at a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

Again, each scenario uses these different themes and development pathways to predict what the level of our GHG emissions is likely to be in the future.

So let’s say you’ve chosen your scenario; what do you do with it? You use it as an input in a climate model. A model is a system of equations that is meant to simulate the dynamics of the atmosphere and/or oceans over time, allowing us to measure how these forces in combination with our emissions will affect our climate. As you might imagine, that gets very complicated very quickly.

There’s two ways of going about putting a model together: a statistics-based approach or a physics-based approach. Statistical models draw on observed data from the past (for temperature, precipitation, and other variables) to predict what comes next, based on what has happened before. They’re very efficient at this, but if the climate is changing in ways we haven’t observed before, they can’t really predict the end results with any real precision based on these unknowns.

Physics-based models, in contrast, are based on our understanding of the laws of physics and how they apply to the atmosphere and ocean. Climate models are primarily physics-based, but may draw on statistical approaches for small-scale processes that may be too challenging for us to model with physics. Either way, it is a lot of math, done by supercomputers in an effort to represent planet Earth in years to come.

GCMs (Global Climate Models or General Circulation Models, depending on who you’re talking to) get particularly specific, breaking down the region being studied over time into discrete cells. Any atmospheric or oceanic processes occurring at a scale smaller than the cell size are defined as parameters (set values) at the cell level. Larger processes are modeled in the interaction between cells. Accordingly, smaller cells = more accurate and detailed predictions:

Source

Look at how Europe-y that last Europe looks! That increased resolution means more accurate, more localized predictions. Still, though, each cell is many miles across. There’s a lot of detail that gets lost even at this level of resolution.

Take clouds, for example. We have basically no idea what the clouds of the future will be like, beyond “different, probably.” As mentioned above, water vapor functions as a GHG; clouds, made of water vapor, therefore have a warming effect on the planet. But clouds are also white; their high albedo causes them to reflect sunlight from above back into space, resulting in a simultaneous cooling effect. Aside from paradoxically warming and cooling at the same time, clouds also occur at very local levels and change rapidly over a very brief timeframe. This makes representing them in models, where regional cells are miles across and timescales are more likely oriented toward years or decades, a challenge. You can’t really ignore clouds in climate models, though, because even small changes in cloud cover can have significant effects on climate. Different models therefore try different methods to simulate them, but the problem of cloud feedback remains largely unsolved for now.

All right, so we’ve fed our emissions scenario that tells us what civilization will look like into our climate model that simulates how the atmosphere and ocean are likely to behave. What comes out? We call that a projection. This is what will happen when the ocean and atmosphere circulate our increasing pollution throughout the planet. This is the vision of soaring temperatures and violent storms that keeps you from clicking on that doomsday headline.

Something to always keep in mind: climate projections depend entirely on the scenario and the model used, and there are many of each. It’s not really possible to say one projection is more accurate than the other, if they each project at the same level of detail, because who is to say exactly which scenario tells us how humanity will behave in years to come?  Food for thought, though: the A2 scenario above, the one that describes a fragmented and divided future world, is sometimes informally called the “business as usual” approach. It more or less reflects how we’re going about things at present, and it also consistently projects grimmer futures. So, uh, yeah humanity, might wanna think about that.

So if we don’t know for sure which is the most accurate, and we’re so limited by so many unknowns, why bother? Because climate change is happening, like it or not. Even if we shut down all our factories (and cows?) tomorrow, the concentrations of GHGs in our atmosphere today are significant enough to continue to alter our future climate. And it’s a much bigger deal than “everyone gets a little hotter.”

Rising temperatures ARE a big deal, of course. For example, many crops only grow within a set temperature and precipitation range. Shifts in regional climate could have significant impacts on agriculture and food supply, with consequent effects on regional stability, population growth and conflict. But rising temperatures also lead to worse natural disasters.

Here’s how: as it gets hotter, the amount of water vapor evaporating into the atmosphere increases. Storms form where areas of low and high pressure meet, created in part due to atmospheric temperature differences. The overall effect of global rising temperatures will be (relatively) smaller differences between the hot equator and frigid poles, and so hotter and thus more uniform global temperatures mean a stable or even decreased risk of storm formation. But those storms that do form are likely to be more intense. Hurricanes, especially, form due to a combination of warm, moist, rising air over a warm sea surface; their power comes from the evaporation of water to vapor. All of these conditions are set to increase with climate change. What I’m saying is, get ready for some crazy hurricanes.

If I want you to come away with anything from this piece, though, it’s that you shouldn’t give up and say, “well, we’re doomed,” if you read about a projection that says the collapse of civilization is imminent. Remember, that projection is based on a possible future, and possible futures can change to an extent by our efforts, particularly if climate policy, energy use or economic development changes. Check the original study and see what scenario they used! It’s something worth thinking about. I’m not very starry-eyed that we’re all going to join hands tomorrow and fix the planet once and for all; I just want to help inform the conversation, because it’s one that needs to continue if we are going to choose a better future.