LITTLETON, Colorado, July 1 (Reuters) - U.S. power sector emissions are already at their highest levels in three years, but will likely climb further during the peak summer months as greater use of air conditioning systems drives higher generation from coal and natural gas plants.

Over the first five months of 2025, U.S. power sector emissions from the burning of fossil fuels were up 5% to around 640 million metric tons, according to data from Ember.

The roughly 32 million ton rise in emissions from the same months a year ago stems mainly from higher use of coal within the U.S. generation mix, as power firms have so far cut back on natural gas use from a year ago after gas prices rallied.

However, power firms are starting to dial up generation from both coal and gas in order to meet higher electricity demand from homes and businesses tied to the greater use of power-hungry air conditioners.

Those higher generation trends will in turn further lift power sector pollution totals, even as electricity production from clean power sources such as solar farms hit record highs.

COAL-HEAVY

Over the first half of 2024, U.S. coal-fired power generation climbed by 14% from the same period in 2024 to 14.9 million megawatt hours (MWh), according to data from LSEG.

The chief driver behind the rise in coal use was a steep rise in the price of natural gas during the opening quarter of the year, which applied fresh cost pressure on utilities and spurred higher use of cheaper coal within generation mixes.

Henry Hub natural gas futures - the main benchmark price for U.S. natural gas - have averaged around $3.53 per million British thermal units (BTU) so far this year, LSEG data shows.

That compares to an average of $2.15 per BTU during the first half of 2024.

As a result of that over 60% jump in gas costs, gas-fired power production during January to June was down 4.2% to 31.8 million MWh, according to LSEG.

The higher proportion of coal power within the U.S. generation mix has had a big impact on overall emissions.

Coal-fired power stations emit roughly 950,000 metric tons of carbon dioxide per terawatt hour of electricity production, according to Ember.

That compares to around 540,000 tons of CO2 per TWh from gas-fired plants, and explains why overall fossil fuel emissions have climbed much more steeply than fossil fuel power output so far this year.

PEAK PERIOD

The U.S. has two well-defined peak periods of power use every year - for heating during winter and for cooling during summer.

And for more than a decade, power use during the summer has exceeded the power needs over the winter, as air conditioning units require greater volumes of electricity than heating systems.

This year that trend looks set to be extended after several parts of the U.S. were gripped by record-setting heat waves during the latter half of June, and are forecast to get further hot spells during July, August and into September.

To meet the resulting rise in electricity use, utilities will need greater power output from all production sources, but especially from fossil fuels which are needed to meet the lion's share of system use at night when solar generation stops.

And as gas prices remain well above year-ago levels, most power generation systems will continue to prioritize lifting output from relatively cheaper coal rather than costlier gas.

That sets the stage for a fresh climb in power emissions, which are already at their highest since 2022 and are primed to hit their annual peak over the coming months as power firms deploy all the power they can muster to keep up with demand.