Emissions and air quality

Changes in energy demand associated with warmer temperatures are driven by the distribution of temperatures at hourly or even sub-hourly scales. Fig 2 shows a histogram of regional (eastern US) average hourly temperatures over the month of July for current and mid-century conditions. Results show a shift in the maximum ambient temperature from 32.4 °C (present) to 38.5 °C (future), an 18.8% increase. The mid-century scenario exhibits a decrease in the frequency of colder temperatures and an increase in the frequency of warmer temperatures.

The higher temperatures seen in the mid-century scenarios drive changes in electricity demand, production, and associated emissions. Fig 3 shows the hourly distribution of electricity production and emissions for current and future climates. These results show the response of electricity production to ambient temperature through demand for air conditioning. Under the future climate assumptions, regionally summed average hourly electricity demand increases from 213 to 274 GWh (28.6%), and regionally summed average hourly eastern US CO 2 emissions increase from 169,000 to 200,000 metric tonnes (18.3%). Thus, adaptation through air conditioning use also constitutes a positive climate feedback.

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 3. Histograms of hourly electricity production and emissions. Histograms are provided for regionally summed hourly electricity production, CO 2 emissions, nitrogen oxide (NO X ) emissions, and SO 2 emissions for July in the present-day and warm mid-century warm climate scenarios. For electricity production: present-day mean: 212.9 GWh; minimum: 120.4; maximum: 320.3. Mid-century mean: 274.2 GWh; minimum: 172.0; maximum: 438.0. For CO 2 emissions: present-day mean: 168,800 tonnes; minimum: 99,800; maximum: 238,800. Mid-century mean: 200,100 tonnes; minimum: 132,700; maximum: 276,500. For NO X emissions: present-day mean: 140 tonnes; minimum: 80; maximum: 210. Mid-century mean: 160 tonnes; minimum: 100; maximum: 210. For SO 2 emissions: present-day mean: 430 tonnes; minimum: 250; maximum: 610. Mid-century mean: 500 tonnes; minimum: 300; maximum: 590. https://doi.org/10.1371/journal.pmed.1002599.g003

The change in maximum CO 2 is not as large as the change in electricity production because additional capacity in mid-century (necessary to meet increased demand) is generated by natural gas power plants based on the US EIA’s Annual Energy Outlook, which emit less carbon than the current mix of generation sources [36]. We find that electricity production and emissions in the present-day exhibit a more uniform distribution than does temperature (Fig 2). This difference is due to the changing sensitivity of electricity generation as a function of temperature, with responsiveness increasing at higher temperatures and decreasing at cooler temperatures, when building cooling is less important. The distribution becomes less uniform in the mid-century climate as temperature dependence plays a greater role compared to other end uses of electricity.

Trends in the distribution of hourly electricity production and CO 2 emissions more closely follow changes in temperature than do emissions of NO X and SO 2 , as shown in Fig 3. Overall, emissions in the future climate scenario increase 13.7% for NO X and 17.2% for SO 2 , but the maximum hourly emissions rate does not increase for either NO X or SO 2 . Rather, the increase in average hourly emissions of NO X and SO 2 occurs from greater frequency of emissions on the higher end of the present-day emissions distribution. Even as electricity demand increases, new peak electricity demand in the model is met by natural gas power plants that have little impact on NO X and SO 2 emissions during peak conditions. Simulating likely retirements of coal-fired power plants and market-driven renewable energy investments would also result in lower emissions than found here, where we maintain the existing power plant inventory to explore the arising interactions between climate, energy production, and air quality without being predictive. This highlights the importance of considering cleaner energy sources in reducing future harmful emissions.

Overall, a 3.5 °C warmer summer is responsible for an increase in hourly average building energy demand of 28.6%. The air conditioning adaptation response to climate change in the eastern US is thus responsible for hourly average emissions increases of 13.7% for NO X , 17.2% for SO 2 , and 18.5% for CO 2 .

We analyzed air quality in the PD (present-day climate, present-day EGU emissions), MCCO (mid-century climate only), and MCA (mid-century adaptation) scenarios as described in the Methods. On a regional average, we find that climate change alone (MCCO versus PD) increases PM 2.5 by 58.6% (2.50 μg/m3) and O 3 by 14.9% (8.06 parts per billion by volume [ppbv]). A larger change is found when comparing the present day to the mid-century adaptation scenario, which includes building air conditioning (MCA versus PD). In that case, PM 2.5 increases 61.1% (2.60 μg/m3) and O 3 increases 15.9% (8.64 ppbv). Overall, 2.5% of the 61.1% increase in PM 2.5 and 1.0% of the 15.9% increase in O 3 are attributable to adaptive behavior (extra air conditioning use).

The July average change in each pollutant due to building energy use is shown in Fig 4 for PM 2.5 (Fig 4a) and MDA8 O 3 (Fig 4b). Increases in PM 2.5 from the MCCO to the MCA scenario (Fig 4a) are highest (as high as >5%) in and downwind of the Ohio River Valley, coincident with the highest concentration of fossil fuel, especially coal-fired, power plants and the greatest increase in EGU emissions. A small decrease (<2.5%) in concentrations is observed in the southeast, centered over South Carolina and the Chesapeake Bay. This is primarily due to a decrease in emissions in these regions (as seen in Fig 5) associated with power plant dispatch changes (see Meier et al. [36]).

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 4. Change in ambient air pollution concentrations. Maps of the percentage change in (a) PM 2.5 and (b) O 3 from the warm mid-century climate-only (MCCO) scenario to the warm mid-century adaptation (MCA) scenario. Red shows concentrations that are greater in the MCA scenario compared to MCCO, while blue shows a decrease in concentrations compared to MCCO. Axes show latitude and longitude. https://doi.org/10.1371/journal.pmed.1002599.g004

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 5. Change in emissions by state. The state by state changes in nitrogen oxide (NO X ) and SO 2 emissions from the present-day (PD) to mid-century (MC) as an absolute value (designated by the bars) and as a percentage (as listed). https://doi.org/10.1371/journal.pmed.1002599.g005

We examined the distribution of regional average concentrations as a function of air pollution level in Fig 6. The number of hours with pollution at the highest levels increases due to climate change alone, and further rises given greater emissions of NO X and SO 2 associated with higher climate-induced electricity demand. For PM 2.5 , the minimum regional average concentration simulated under a future climate (4.37 μg/m3 for MCCO) is above the average value for present-day (4.26 μg/m3). Present-day values range from a minimum of 2.91 μg/m3 to a maximum 5.98 μg/m3. The highest regional average concentrations modeled under a future climate (8.75 μg/m3 for MCCO) are higher than we see at any time in the present-day simulation. The additional consideration of adaptation through air conditioning use further increases the minimum and maximum values to 4.48 μg/m3 and 8.87 μg/m3, respectively.

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 6. Histograms of ambient air pollutant concentrations. Histograms of regional average hourly concentrations of PM 2.5 (μg/m3) and O 3 (parts per billion by volume [ppbv]) for July in the present-day (PD) scenario, the warm mid-century climate-only (MCCO) scenario, and the warm mid-century adaptation (MCA) scenario. For PM 2.5 concentrations: PD mean: 4.19 μg/m3; minimum: 2.91; maximum: 5.98. MCCO mean: 6.57 μg/m3; minimum: 4.37; maximum: 8.75. MCA mean: 6.67 μg/m3; minimum: 4.48; maximum: 8.87. For O 3 concentrations: PD mean: 43.4 ppbv; minimum: 23.6; maximum: 61.7. MCCO mean: 48.0 ppbv; minimum: 26.4; maximum: 70.2. MCA mean: 48.4 ppbv; minimum: 26.6; maximum: 70.9. https://doi.org/10.1371/journal.pmed.1002599.g006

Biogenic emissions, enhanced under a warmer climate, are the dominant contributor to the MCCO increase in PM 2.5 . This impact is sensitive to the choice of chemical mechanism in the atmospheric model and details regarding the formation of secondary organic aerosol as a function of volatile organic compounds. Past studies have suggested that the CB05 mechanism in CMAQ may have errors in the representation of this atmospheric chemical process [91–93]. Thus, while the direct impact of climate on PM 2.5 is notable, we focus our discussion on the changes due to building energy use (i.e., MCCO versus MCA).

Modeled EGU emissions of SO 2 increase by 17.2%, and NO X by 13.7%, due to building energy use in the future climate (state-by-state variation shown in Fig 5). This increase in EGU emissions results in increases in sulfate particulate matter (SO 4 2−, 5.8% as compared to MCCO, or 0.09 μg/m3) and nitrate PM (NO 3 −, 3.1% as compared to MCCO, or 0.7 × 10−3 μg/m3).

Ozone exhibits many of the same patterns as exhibited by PM 2.5 . However, the increase in hourly O 3 is not as pronounced from the present-day to mid-century scenarios as seen for PM 2.5 . In the case of O 3 , adaptive behavior is responsible for an approximately 1% increase in O 3 . Like PM 2.5 , O 3 increases across most of the region (Fig 4), with the greatest increases in and downwind of the Ohio River Valley (as high as >5%) due to increases in EGU NO X emissions. Small decreases due to localized emissions decreases from changes in electricity dispatch are also evident over South Carolina and Chesapeake Bay, as well as a highly localized decrease in Maine and a very small decrease along the Texas domain boundary.