LITTLETON, Colorado - The month of July kicks off the annual peak period for power emissions in the United States, as higher nationwide temperatures ratchet up use of energy-intensive air conditioners and spur power firms to crank up output from fossil fuels.

The July-to-September quarter has marked the quarterly high point for fossil fuel power output over the past three years, with fossil-fired generation rising by an average of 37% from the previous quarter since 2021, data from LSEG shows.

Resulting emissions during the third quarter of the calendar year have jumped by a similar degree, according to energy think tank Ember, and have again have historically scaled their annual peak during the July-September period.

U.S. power emissions through the first half of 2024 have already climbed by 5.2% from the same period last year, to 790 million metric tons of carbon dioxide (CO2) and equivalent gases, according to Ember.

If power firms follow their normal practice and rely on fossil fuels to plug most of any power supply shortfalls during this year's peak demand period, 2024's emissions toll will likely climb even further above last year's total.

GASSING UP

Power firms traditionally rely on natural gas to supply the majority of any incremental rise in power usage during the third quarter, with gas-fired output rising by an average of 36% since 2021 during that time window from the second quarter.

However, coal-fired power generation also tends to climb sharply during the July to September period, and in 2023 rose by more than 50% from the second quarter as power firms struggled to meet the higher power consumption during that period.

A key driver behind the heavy reliance on fossil fuels during the height of summer is the tendency for clean power supplies to hold flat or even contract slightly during that period.

Clean power generation tends to slump during the summer due to lower wind speeds at turbine level and reduced hydro dam generation during the driest months of the year.

In 2023, clean power generation expanded by just 0.4% during the third quarter from the second quarter, LSEG data shows.

In 2022 and 2021, clean generation declined by 4.3% and 1.7% respectively during that quarter from the previous quarter.

TOUGH CHOICES

The mixed track record of clean power generation right when total power demand reliably peaks leaves U.S. power firms with little scope but to boost output from fossil fuels despite ongoing commitments to decarbonize power production.

Power sector emissions tend to undergo significant jumps whenever power firms raise the share of fossil fuels in the overall generation mix.

Between 2019 and 2023, average power sector emissions during the third quarter have been 532 million tons, according to Ember.

That compares to 401 million tons for each of the other quarters of the year, and highlights the scale of the environmental impact from greater fossil fuel use.

However, the July to September window is also when the U.S. power system tends to come under strain from heat waves and heavy storms that historically cluster during the hottest time of year.

To ensure round-the-clock power availability and meet higher baseload requirements from greater commercial and residential air conditioner use during summer, most power systems are obligated to boost overall output by whatever means necessary.

Fossil fuels have formed the backbone of that additional power generation in recent years, and have resulted in a jump in associated emissions which may worsen the climate-warming trends that have lifted air conditioner demand in the first place.

But if power firms continue to build out clean generation and battery storage capacity at the record pace seen over the past decade or so, clean power sources may be able to replace some fossil fuels in power generation mixes in the years ahead.

<The opinions expressed here are those of the author, a columnist for Reuters.>

(Reporting by Gavin Maguire; Editing by Lincoln Feast.)