Even simple models can help us understand how the rate of carbon dioxide emissions relates to climate change. The basic model below, called the Very Simple Climate Model, gives us a peek into the future. You suggest the rate that you think humans will release CO2 into the atmosphere in the future and the model calculates what that means for atmospheric CO2 concentration and global temperature.
Models let you mix up scenarios for how things could be and see what could happen. Below are three scenarios. Test them out on the Very Simple Climate Model and see for yourself what the future may hold.
Scenario #1: What if we can reduce the rate that we add carbon dioxide to the atmosphere to 9 gigatons of carbon per year (9 GtC/yr)?
- Set the carbon dioxide emissions rate to 9 GtC on the bar under "Select an emission rate".
- Check the play button at the bottom of the left side. What do you notice in the graph?
- Notice how the carbon emissions make a horizontal line since the rate stays the same over time. Do you see the CO2 concentration increasing? Adding CO2 at a steady rate causes the concentration to climb. Take a look at the temperature line on the graph. In this model run, how much did the temperature rise by the year 2100? Is it above the recommended limit?
Scenario #2: What if our emissions get worse before they get better?
The glass is half full! People will rise to the challenge and limit the emissions of greenhouse gases in the atmosphere. Sure, it might take some time, but we will have new technologies and policies in place to limit the amount of carbon dioxide released. In this scenario, let’s assume that people will keep emitting carbon dioxide at an increasing rate until 2050, and then we will have figured out how to stop polluting and the rate will fall.
- Click the “Start Over” button to clean the slate.
- Start with carbon emissions set at 10.5 GtC/yr, at or close to the current rate.
- Click the "Step Forward" button (lower left) twice to advance "model time" by 10 years.
- Increase emissions by 1 GtC/yr (to 10 GtC/yr) and advance another 10 years.
- Continue to increase the emission rate by 1 GtC/yr (to 10 GtC/yr) for each decade, assuming it will take some time before we are able to take action, until you reach the year 2050.
- Once you reach the year 2050, start decreasing the emission rate by 1 GtC/yr every 10 years.
- Continue to decrease the emission rate by 1 GtC/yr for each decade until the model gets to the year 2100. What do you notice about the temperature?
- Did you notice that emissions (blue) increased until 2050, then decreased through 2100? Did you see that CO2 concentration in the atmosphere (black) continued to rise even as emissions dropped? That’s because we were still pouring CO2 into the atmosphere, just at a slower rate. Temperature (red) follows a trend similar to that for CO2 concentration - continuing to rise even as emissions drop, but at a slower pace.
Scenario #3: What does it take to stay below 2°C?
Do you see the red line labeled "recommended temperature limit?" What level of CO2 emissions do we need to stay within that limit during the 21st Century?
- Start with carbon emissions set at 10.5 GtC/yr, close to present-day levels.
- Play the simulation until the temperature line (red) crosses the recommended limit. Mouse over the data point to find out what year that would be.
- Click the "start over" button to try again.
- Select a different emissions rate that you think will keep temperature below the limit this century.
- Play the simulation and see if it was enough.
Some Notes About This Model
- A major educational point embodied in this model is that temperatures depend on concentration, which rises whenever emissions are greater than zero. You've probably heard sound bites along the lines of "we are working hard to reduce the rate of growth of greenhouse gas emissions", presented as if such an approach would eventually lead to reduced temperatures.
- This model is very, very simple. It knows nothing of changing wind or precipitation patterns that might accompany and in turn influence warming; it doesn't care where in the atmosphere the CO2 is; it ignores other greenhouse gases; and so on. In this simple model, the temperature is determined entirely by the atmospheric CO2 concentration via greenhouse warming of the atmosphere.
- While the assumptions behind this model are limited, they are valid. The starting values for concentration, emission rate, and temperature are the actual values for the year 2015. The relationship between atmospheric CO2 concentration and temperature uses a well-established relationship; basically, temperature rises about 3° C for each doubling of CO2 concentration (the climate change sensitivity). So, for example, if the concentration goes from 400 ppmv to 800 ppmv, we expect to see temperature go up by 3° C.