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The amount of carbon dioxide (CO₂) in the atmosphere is increasing. The world is getting warmer. If this continues, the ecosystems and economies of the world will be dramatically altered. What can be done about this?

Click on the picture above to launch a simulation that lets you decide the future of the planet. We use water in a bathtub to represent CO2 in the atmosphere. Can you keep the bathtub from overflowing?

This simulation was designed in collaboration with The Sustainability Institute and the Society for Organizational Learning. The numbers that drive the graphs and the bathtub animation were calculated in a system dynamics model built by Dr. Thomas Fiddaman. His research can be viewed here.  

Read the information below to find out more about CO₂, climate change, and the way we designed this simulation.

CO₂ and Climate Change

We know that carbon dioxide (CO₂) is increasing in the atmosphere from human activities such as burning of fossil fuels and deforestation. This increase is one of the major factors in global warming. There is no longer any scientific debate about this. The most recent report by the Intergovernmental Panel on Climate Change has confirmed this.

In 2007 the concentration of CO₂ in the atmosphere is approximately 380 parts per million (ppm). Every year human activities add to that. Some scientists and economists in the climate science world such as Nicholas Stern and James Hansen (note) have identified a concentration of 450 ppm as a maximum goal for CO₂ that may avoid the most significant damage to the Earth's ecosystems and economies. There is a great deal of uncertainty about the severity of the effects associated with this or any other target level for CO₂.  We have chosen to use it for this simulation, but we could have set it higher or lower. As you play with the simulation consider how the three scenarios would play out if the bathtub overflowed at a level other than 450 ppm.

Already there is much more CO₂ in the atmosphere than at any time in the past 425,000 years. Follow the green line to the right edge of this graph. This is where we are today.

We cannot change what has already happened—the gray parts of the graphs in our simulation show the past from 1950 to 2007. Now the challenge is to find a way to decrease the CO₂ emissions into the atmosphere.
The question is, by how much? This simulation challenges you to choose a future for human emissions of CO₂. The top of the bathtub is 450 ppm of CO₂. The goal: To keep CO₂ in the atmosphere below the level of 450 ppm. But what is the best way to do this? Should we keep letting emissions increase? Should we keep emissions at the current level? Or should we reduce emissions?

The principle at work here is stock and flow. A “stock” is something that accumulates, in this case CO₂ in the atmosphere, represented by the water. The bathtub stands in for the Earth’s atmosphere. Water (CO₂) enters the bathtub (atmosphere) from the spigot above and leaves the bathtub through the drain below. This is the “flow,” a representation of how much goes in and how much goes out.

For the past 425,000 years the amount of CO₂ in the atmosphere has fluctuated between 175 ppm and 300 ppm. The inflow (the amount of CO₂ going into the atmosphere) and outflow (the amount of CO₂  removed from the atmosphere) were sufficiently in balance during this period of time to keep the CO₂ level within that range.  In the past few decades the inflow has increased dramatically. The flows are now out of balance. More and more CO₂ is entering the atmosphere, but not nearly as much is being removed. Thus, CO₂ increasingly accumulates in the atmosphere. The amount now stands at a concentration of 380 ppm. In our simulation, the bathtub can overflow if the amount of CO₂ in the atmosphere increases to the point of significantly altering the climate.

So what do you think we should do? Play the game to see if you can figure out how to stop global warming. Here are your choices:

• Allow Increasing CO₂ Emissions
One scenario is to allow human emissions of CO₂ to increase at roughly current levels. This means that governments around the world would not regulate CO₂ emissions, and businesses and individuals would not take any special action to reduce CO₂ emissions. Everything would continue on as it has been going. This “business as usual” or “status quo” approach asks: What if we did nothing?

The numbers used in this scenario were the “business as usual” estimates of the Intergovernmental Panel on Climate Change (IPCC), the international group dedicated to studying this issue. In this scenario, removals increase naturally, but are never able to keep up with the increase. By the year 2045 the levels would reach 450 ppm. This amount would cause significant changes in the atmosphere, and global warming would cause dramatic changes to the environment.

In our climate simulation, the bathtub would overflow by 2045, and we would experience even more significant climate change. This future is what the IPCC scientists expect will happen if we make no major changes to avert climate change.

• Level Off CO₂ Emissions
Another option is to gradually stop the increase of human-caused emissions of CO₂ in the decades following 2007.  This scenario is based loosely on the Kyoto Protocol, an international treaty to reduce CO₂ emissions. The treaty was negotiated by the United Nations Framework Convention on Climate Change (UNFCCC) in 1997 and went into effect in 2005. More than 150 nations were involved in creating the Kyoto Protocol, and 84 countries signed the agreement. However, the agreement also needed to be ratified by each country, and not all who signed the protocol ratified or approved it at home. The leading industrialized countries that have ratified it include Russia, Japan, and the members of the European Union. Other countries have since joined the agreement, bringing the total to more than 165.  The United States and Australia are among the industrialized countries that signed but did not ratify the Kyoto Protocol.

The countries that did agree to follow the protocol produce about 60% of the world's greenhouse gases. The agreement is for industrialized countries to reduce greenhouse gas emissions to 5.2% lower than 1990 levels by 2012. This would roughly level off CO₂ emissions. But is stabilizing emissions enough to prevent CO₂ levels from going above 450 ppm?

• Reduce CO₂  Emissions
What if all the governments in the world agreed to significantly reduce CO₂ emissions? A plan like this has been proposed by former U.S. vice president Al Gore. Climatologist David Stern has proposed something similar. This scenario calls for reducing emissions of CO₂ by 58% of the 2007 level by 2070. What would happen to our bathtub? Would it still overflow?

Questions about CO₂ and Climate Change

What are CO₂  emissions?
Carbon dioxide (CO₂) is a gas that makes up a tiny fraction of the Earth’s atmosphere. It occurs naturally, mostly as a result of breathing, of decay, from the burning of wood and the release of CO₂ from the oceans. CO₂ emissions also result from the burning of fossil fuels and other human activities.  It is this human-generated CO₂ that we are showing in our simulation.

What are CO₂ removals?
Carbon sinks remove carbon from the atmosphere. The main carbon sinks responsible for removals are photosynthesis and absorption by the oceans.

The oceans are both a carbon sink and a source of CO₂. There is an ongoing exchange of CO₂ between the atmosphere and the oceans. The balance depends upon factors including water temperature and the concentrations of CO₂ in both the oceans and the atmosphere.

For hundreds of thousands of years emissions and removals remained roughly in balance with the concentration of CO₂ in the atmosphere varying between 180 and 300 parts per million (ppm). This was true until humans began to burn fossil fuels during the Industrial Revolution. These additional CO₂ emissions are the problem. Currently much more CO₂ is being released than can be taken up by plants or absorbed by the ocean. The concentration of CO₂ in the atmosphere is now 380 ppm and rising.

Why do removals seem to follow emissions?
Carbon dioxide flows between the atmosphere, biosphere, and oceans in order to maintain a balanced distribution. When the concentration of CO₂ in the atmosphere increases, two things happen:

• "CO₂ fertilization" occurs. Plants use more CO₂ for photosynthesis, growing more leaves and woody material.
• The surface ocean—mixed by wind-driven waves— quickly absorbs CO₂, which then diffuses more gradually into the deep ocean.

Both processes have limits. The oceans can only absorb so much CO₂ before releasing as much CO₂ back to the atmosphere as was taken up. For plants, the limitations on growth from water and other nutrients become important. This is called “sink saturation.”

In the "Allow Increased Emissions" future, removals increase because the rapidly-growing concentration of CO₂ in the atmosphere continues to drive uptake. Part of the excess CO₂ is absorbed by plants and the oceans.

In the "Reduce CO₂ Emissions" future, removals fall because the excess of CO₂ in the atmosphere above that in the biosphere and oceans is not so great.

What’s the connection between CO₂  and climate change?  
We know that CO₂ absorbs heat from the Sun and releases it into the atmosphere. Going back millions of years, when the concentration of CO₂ was higher, the Earth was warmer. Eventually CO₂ concentration dropped and the world became cooler. Since the 1740s CO₂ concentration has increased significantly, and the average temperature on Earth has also increased.

Why does the CO₂ level in the atmosphere continue to rise even when emissions are leveled off?
This scenario corresponds to clicking the middle button in our simulation: "LEVEL OFF CO₂ EMISSIONS." After about 2045 emissions are no longer increasing. At that point removals are also level from year to year. But since emissions are greater than removals, each year more CO₂ goes into the atmosphere than is removed. So the amount of CO₂ in the atmosphere continues to rise. 

It’s like a bus traveling through the city with people getting on and off. Let’s say that at one stop 5 people get on the bus and 3 get off. At the next stop the same thing happens: 5 people get on and 3 get off. If this pattern continues the bus will get very crowded. The number of people getting on the bus is level: 5 at each stop. But since only three people get off there is an increase of 2 people each time the bus stops. In order to keep the crowding from getting worse, the same number of people have to get off the bus as get on.  And to reduce the crowding, more people have to get off than get on.

In order to keep the concentration of CO₂ in the atmosphere at a given level, say 450 ppm, emissions and removals have to be equal.  In order to reduce the concentration of CO₂ in the atmosphere, removals have to be greater than emissions.

¹Hansen, James, et. al., 2006: Dangerous human-made interference with climate: A GISS modelE study. Atmos. Chem. Phys., submitted.