ISO Express chart estimates power system carbon emissions in real time

This is the latest installment in a series highlighting data visualizations created by ISO New England to help explain different aspects of the region’s bulk electric system.

New England relies on a variety of resources to generate the electricity it needs, including some resources that also produce carbon emissions. To help keep the region informed about power system emissions, ISO New England provides estimates annually, monthly, and in real time.

The Estimated CO₂ Emissions chart, part of the ISO Express data dashboard and ISO to Go mobile app, presents estimates throughout the course of each day.

The red line, highest on the chart, shows the sum of estimated CO₂ emissions from all resource types. Each of the lines below it show estimated emissions from a specific resource type: natural gas (light blue), landfill gas (green), wood (light brown), refuse (dark brown), oil (light gray), and coal (dark gray).

The estimates are based on emission factors, derived from US Environmental Protection Agency data and updated annually, for each fuel type. The real-time output from each fuel type, measured in megawatts, is multiplied by its corresponding emission factor to produce an estimate of metric tons of CO₂ emitted. Values update throughout the day each time ISO-NE system operators adjust the way the region’s fleet of energy resources is dispatched.

Natural gas has the lowest emission factor (0.00654 metric tons of CO₂ per megawatt per minute) of all carbon-emitting fuels used to generate electricity in New England. But because it is also the region’s most prevalent fuel type, emissions from natural gas tend to outpace emissions from other fuel types by a wide margin.

In contrast, coal has the highest emission factor (0.02377 metric tons of CO₂ per megawatt per minute). But coal-fired resources operate far less frequently, and so their share of actual emissions is considerably smaller. In 2024, these resources accounted for about 1% of the estimated total power system carbon emissions.

Today’s power system emissions are significantly lower than those observed in previous decades. Carbon dioxide emissions declined 20% between 2015 and 2024, and have fallen even more significantly since the early 2000s.

New England also relies on several other kinds of energy resources, including nuclear, hydro, wind, solar, and energy storage devices. But these resource types do not appear on the Estimated CO₂ Emissions chart because they do not produce carbon emissions.

The chart does not include emissions estimates for electricity imported from neighboring areas because the resource types producing this electricity are unspecified. The ISO does estimate import-related emissions on an annual basis in its Electric Generator Air Emissions Reports.

The annual reports also provide a more detailed analysis of native carbon emissions, as well as sulfur dioxide (SO₂) and nitrogen oxides (NO_x), which are emitted by generating resources in much smaller quantities than CO₂.

In addition to the real-time estimates and annual analysis, ISO-NE publishes data on estimated CO₂ emissions from New England power plants in a monthly recap of the wholesale electricity markets.