# Part 2 of 4: Worldwide Emissions

*Published:* 2007-03-14
*Author:* Bill Chameides

*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?](https://blogs.edf.org/climate411/2007/03/07/tipping_point/)*  
*2. Worldwide Emissions Target*  
*3. [U.S. Emissions Target](https://blogs.edf.org/climate411/2007/03/21/us_emissions/)*  
*4. [Technologies to Get Us There](https://blogs.edf.org/climate411/2007/03/28/green_technologies/)*

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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](https://blogs.edf.org/climate411/2007/03/07/tipping_point/). 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](http://www.realclimate.org/figure1_hansen05.jpg). 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.

![](/climate411/wp-content/files/2007/03/co2_concentrations.png)

*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](http://www.sciencemag.org/cgi/content/summary/315/5817/1371?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&searchid=1&FIRSTINDEX=0&volume=315&firstpage=1371&resourcetype=HWCIT)). 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](http://www.whitehouse.gov/news/releases/2007/01/20070123-2.html).)

![](/climate411/wp-content/files/2007/03/annual_co2_emissions.png)

*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.