Part 2 of 4: Worldwide Emissions

This is the second installment of a four-part series to be published each Wednesday on Action Needed to Stop Global Warming.

1. How Warm is Too Warm?
2. Worldwide Emissions Target
3. U.S. Emissions Target
4. Technologies to Get Us There

In Part 1 of this series, we described why scientists and policy-makers have identified the melting of the Greenland ice sheet, which would lead to a 20-foot rise in sea level, as the tipping point that must not be crossed. To stay below the tipping point, average global temperatures must not rise more than 3.6oF above pre-industrial temperatures, or 2.3oF above current temperatures.Today we consider how global emissions of greenhouse gases must change over the coming century to stay below that tipping point.

To answer this question, scientists have developed models – mathematical descriptions of the relationship between greenhouse gas concentrations and global warming. To verify the validity of the models, scientists check how well they do in predicting what’s happened in the past. They do a good job. Since the models can accurately describe past climate patterns, they should be able to do a reasonable job describing future climate patterns.

The most important of the greenhouse gases is carbon dioxide (CO2), because its rising concentration – caused primarily by the burning of fossil fuels – is having the largest single impact on climate change. Model calculations indicate that to avoid a temperature increase of 3.6oF, we must stabilize CO2 concentrations at about 450 parts per million (ppm) or less.

Source for graph: IPCC Third Assessment Report (2001).

This turns out to be a tall order. Today, CO2 concentration is 380 ppm. The rate of increase is about 2 ppm per year, and is expected to accelerate. If we follow a “business as usual” course, we could cross the 450 ppm tipping point well before 2050.

Naturally, the more CO2 we produce, the higher CO2 concentration in the atmosphere. Four gigatons of carbon emitted into the atmosphere will raise CO2 concentrations by 1 ppm (see CO2 Arithmetic, Science Magazine). Worldwide, 7 to 8 gigatons of carbon are emitted into the atmosphere each year.

To avoid the tipping point, global CO2 emissions should peak no later than 15 years from now, and then begin to decrease. By 2050, emissions must be about 50 percent less than today, and by the end of the century 75 percent less. (Note that this is a reduction in total emissions, not the reduction relative to projected business-as-usual emissions that President Bush referred to in his 2007 State of the Union address.)

Source for Graph: Confronting Climate Change, United Nations Foundation

Reducing CO2 emissions by 75 percent will require a profound change in the way we produce and use energy, but there is no need for panic or despair. If we get started now, we can make this transition slowly, a percent or two each year. It’s a job that our children and grandchildren will continue to work on through the end of the century, but we can start today. As we will see in Part 4 of this series, the technologies we need are already in hand.

This entry was posted in Basic Science of Global Warming, Greenhouse Gas Emissions. Bookmark the permalink. Both comments and trackbacks are currently closed.


  1. Enrique
    Posted March 14, 2007 at 11:17 am | Permalink

    What is the relation between the accumulation of CO2 and the destruction of the ozone layer?
    Is there one? or is that another problem?


  2. Posted March 14, 2007 at 12:41 pm | Permalink

    Good question and one that I get often.

    Climate change from CO2 pollution and destruction of the ozone layer are basically unrelated, but there is one way they do interact. Greenhouse gases like CO2 raise the temperature in the lower atmosphere, but lower it in the upper atmosphere or stratosphere, where the ozone layer is.

    I know that sounds counterintuitive. Here’s why it happens. Heat leaves the earth by radiating into space, and CO2 is one of the radiators. With higher CO2 concentrations, the altitude at which the CO2 radiates to space increases and this causes cooling and lower temperatures in the stratosphere.

    The cooling in the stratosphere leads to the creation of more ice crystals. Since reactions that deplete the ozone occur on the surface of these ice crystals, higher CO2 concentrations can exacerbate ozone depletion.

    By the way, while we’re on the topic, the rate of increase in the ozone hole is slowing, in large part because of the international treaties that have banned to use of chlorofluorocarbons. It’s hard to tell for sure because of year-to-year fluctuations, but it may even have started to decrease. It will take at least another 10 years or so for the hole to be gone completely.

  3. deathsinger
    Posted March 15, 2007 at 11:36 am | Permalink

    Bill, you wrote:

    “It will take at least another 10 years or so for the hole to be gone completely.”

    Please check the 2006 World Meteorological Organization/United Nations Environment Programme Scientific Assessment of Ozone Depletion.

  4. dshepard
    Posted March 15, 2007 at 12:56 pm | Permalink

    By “Harnessing High Altitude Wind Power”, as per the paper just published by the IEEE, the tipping point can be avoided, and I think will be. Please see IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 22, NO. 1, MARCH 2007. One method of capturing this energy is through the use of Flying Electric Generators, as detailed on the website.

    I am glad to see that environmental defense is not advocating ethanol in these articles. As James Hansen of NASA points out, ethanol contibutes to global warming if used in cars. However, if used to generate electricity and the CO2 were to be sequestered in the process, this would actually decrease CO2 in the atmosphere and assist in the battle to head off global warming.

  5. Posted March 15, 2007 at 1:08 pm | Permalink

    Thanks, deathsinger, for keeping me honest. I should have said it will take decades for the hole to be gone (i.e. return to pre-1980 levels).

    The 2006 Assessment:

    projects that the ozone hole will heal between 2060 and 2075, some 10-25 years later than projected in the 2002 Assessment. Moreover, the 2006 Assessment says it will be more than 20 years before we see a significant improvement.

    This is an unfortunate illustration of the pitfalls of waiting too long to respond to warnings of an environmental crisis. If the world had acted in the 1970’s when scientists first warned of the dangers of CFCs, we could have prevented all this. Now we’re at serious risk of making the same mistake with the climate – and the consequences will be far greater.

  6. Posted March 15, 2007 at 2:05 pm | Permalink

    Hi dshepard. High altitude wind power is an interesting idea, and perhaps it can adequately reduce CO2 emissions while meeting our needs for electricity. But there are a lot of good ideas floating around, and there’s no way to know which will pan out. That’s why, rather than backing any particular technology, I think it’s better to get policies in place that will empower folks like you to implement your ideas.

    A cap-and-trade on greenhouse emissions would do just that. It guarantees that emissions will be reduced by a specific amount by a specific date, and encourages innovation and savvy investment by rewarding the technologies that reduce the most emissions at the least cost.

  7. Posted January 16, 2010 at 10:19 pm | Permalink

    I more often than not don’t post in Blogs. Though, your blog forced me to! Awesome work.. Keep it coming! Regards!