The variations in generation and capacity in response to the individual and combined climate impacts suggest that the response of the electricity system to climate change depends on the scale of interest. At the national level, the dominant effect is the increase in generation and capacity driven by increases in electricity demand resulting from higher air temperatures. This effect is robust to the choice of climate model, although the magnitude depends on the magnitude of the temperature response, which depends on the climate model. The impacts of other temperature and water availability-mediated effects on net generation and capacity changes are considerably smaller than the Temp-Demand effect across all climate models.
Since increases in air temperature are ubiquitous, it is intuitive that the Temp-Demand effect would have a significant impact on the electricity system. However, there are two aspects of the national-level results that are less intuitive. One is the amount by which the Temp-Demand effect dominates all other air temperature and water availability effects at the national level. Another is that most of the increase in generation and capacity in the Temp-Demand case is provided by natural gas, even though both wind and solar technologies are assumed to achieve substantial cost reductions over the study period and future natural gas prices are assumed to be higher than current prices.
The dominance of the Temp-Demand effect can be explained by examining the representation of the different effects in ReEDS. Empirical estimates of the sensitivity of load to increases in temperature (Sullivan et al. 2015) that are used in ReEDS generally indicate a 1–2% increase in load per °C increase in air temperature (due to increases in space cooling demand), with a significant amount of regional variability. At the same time, estimates suggest a somewhat smaller sensitivity of load to decreases in temperatures (due to lower heating demand during colder months). Since the average increase in air temperature in 2050 is about 1–2 °C across the models, we might expect a 1–4% increase in generation (50–200 TWh, assuming ~ 5000 TWh in 2050), which is consistent with results shown in Fig. 2.
This Temp-Demand effect on generation can be contrasted with the other temperature- and water availability–mediated effects on generation. None of the other effects change the overall amount of electricity demanded, so at a national level, we would expect the net change in generation from these other effects to be small, which is also consistent with Fig. 2. That said, decreases in the maximum capacity of power plants or transmission lines may increase the need for additional generation capacity even if overall electricity demand—and the need for generation—is unchanged.
However, the impacts on capacity from other effects are likely to be considerably smaller than the impacts from the Temp-Demand effect for three reasons. First, the sensitivity of maximum power plant and transmission line capacity to temperature is less than 1% per °C, which is smaller than the sensitivity associated with the Temp-Demand effect. Second, in the case of Temp-Gen and Temp-Trans, these effects are not likely to be material in some regions because not all regions face binding transmission constraints or binding reserve margin constraints. That is, they have excess generation or transmission capacity, in which case reductions in capacity may not alter the outcome. Third, in the case of Water-Gen effects, water availability may increase in some regions, alleviating rather than exacerbating pressure from this constraint, and, as with the other effects, water constraints are not binding in some regions.
The disproportionate share of natural gas deployed in response to the Temp-Demand effect can be understood by examining the change in the load shape that occurs in addition to the increase in overall load that results from increased air temperatures. Since increases in demand are largely due to space cooling, this impact is concentrated in peak demand time periods—primarily afternoon hours on hot days (Fig. SM 8). Natural gas technologies are deployed to satisfy the additional need for flexible generation, as well as the need for capacity to satisfy reserve margins as the required amount of peak capacity increases. The additional capacity that deploys in this case is a mix of NG-CCs, which satisfy the need for additional generation (Fig. 2, top row), and NG-CTs, which satisfy the need for additional reserve capacity (Fig. 2, bottom row).
From these additional considerations, it is clear why the Temp-Demand effects on both generation and capacity are more significant than the Temp-Gen, Temp-Trans, and Water-Gen effects. In the case of generation, only the Temp-Demand effect increases the overall demand for electricity and therefore the need for additional net generation on a national scale. In the case of capacity, only the Temp-Demand effect increases the peak-to-average ratio and therefore the need for additional net capacity to satisfy reserve margins. The other effects alter the need for capacity in some regions, but do not consistently do so across all regions, leading to smaller changes in net capacity at a national level.Footnote 11
At the regional level, there is a lack of correlation between the electricity system response in a given region and the physical climate response in that region. This effect is most apparent in the left panel of Fig. 3, in which the regional temperature responses are all observed to be positive while the changes in generation (due to the Temp-Demand effect) are both positive and negative by 2050. The national-level results, and the explanation for those results, guarantee that the sum of the regional changes in generation is positive, but the same reasoning does not constrain the regional results in a model in which electricity can be exchanged between regions and in which regional demands for electricity can be satisfied more cost-effectively by increasing generation in one region and decreasing it another. An implication of this result is that, while the changes in the electricity system at the national level can be directly related to changes in climate, changes in the electricity system at the regional level are more likely to be determined by the relative costs of providing electricity within a particular region than by the changes in climate in that region.
Regional results in the Water-Gen scenario (Fig. 3, center panel) largely confirm the Temp-Demand explanation; comparable changes in water availability across different regions may lead to increases in generation in one region, but decreases in generation in another. A key difference, however, is that the projected changes in water availability can be substantially different across models. This result suggests that given uncertainty in regional water availability projections across climate models, regardless of the fidelity of the electricity system representation, water-mediated climate impacts on electricity are not likely to be projected with confidence at the regional level, making scenario and uncertainty analysis of water availability essential to fully understand the range of possible outcomes. That said, the electricity system responses exhibit a general pattern that is similar to the Temp-Demand effect, with some regions having consistently lower generation and some having consistently higher generation when perturbed by a change in climate. This result supports the explanation above that the primary driver of results in a given region is the regional variation in cost and other features of the electricity system, rather than changes in climate in that region. This also explains how some states with relatively large changes in water availability exhibit minimal changes in generation, and states with relatively small changes in water availability exhibit relatively large changes in generation.
Finally, although the Temp-Demand effect tends to drive first-order changes in generation and capacity, interaction effects with other supply-side drivers can alter the mix of technologies in some cases. For example, in the warmer and drier climate scenario, the All scenario exhibits net generation differences comparable to the Temp-Demand scenario, but there is more fuel switching in the All scenario. NG-CC generation differences are 30% higher in the All scenario, presumably a result of the fact that lower water availability constrains coal generation. Conversely, in the cooler and wetter climate scenario, there is less fuel switching in the All scenario than in the Temp-Demand scenario, even though changes in net generation are roughly equivalent.