The United States’s Nationally Determined Contribution (NDC) to the 2015 Paris climate accord committed to reducing greenhouse gas emissions to 26–28% below 2005 levels by 2025 and with a longer-term target of reducing emissions to 80% below 2005 levels by 2050 (United States of America 2015).1 This vignette will compare what these goals imply for rates of improving energy efficiency and transitioning from fossil fuels to clean energy sources. The methods will follow Roger Pielke, Jr.’s approach for both bottom-up and top-down analysis (Pielke 2009b, 2009a, 2010, 2011).
The top-down analysis is very similar to the bottom-up analysis, but instead of looking at the elements of the Kaya identity individually, we use predictions from macroeconomic integrated assessment models that consider interactions between population, GDP, and energy use to predict future energy demand:
<- get_top_down_trends("United States") top_down_trends %>% select(P, G, E) %>% top_down_trends mutate_all(list(~map_chr(., ~percent(.x, 0.01)))) %>% rename("P trend" = P, "G trend" = G, "E trend" = E) %>% kable(align="rrr")
|P trend||G trend||E trend|
In the bottom-up analysis, we calculated the implied rate of decarbonizing the economy by comparing the rate of emissions reduction implied by the policy (\(r_F\)) to the predicted rate of change of GDP (\(r_G\)). Here, in the top-down analysis, we calculate the implied rate of decarbonizing the energy supply (\(r_f\)) by comparing the rate of emissions-reduction implied by policy (\(r_F\)) to the predicted rate of growth of energy demand (\(r_E\)): \(F = E \times f\), so \(r_F = r_E + r_f\), which we rearrange to find that \(r_f = r_F - r_E\).
<- F_target %>% implied_decarb_rates_top_down transmute(year, impl_F = implied_rate, top_down_E = top_down_trends$E, hist_f = historical_trends['f'], impl_f = impl_F - top_down_E) %>% implied_decarb_rates_top_down mutate_at(vars(starts_with("hist_"), starts_with("impl_"), starts_with("top_down")), list(~map_chr(., ~percent(.x, 0.01)))) %>% select(Year = year, "implied F" = impl_F, "top-down E" = top_down_E, "implied f" = impl_f, "historical f" = hist_f %>% ) kable(align="rrrrr")
|Year||implied F||top-down E||implied f||historical f|
To meet the goals for 2025 would require increasing the rate of reducing f from -1.08% per year to -1.81% per year: 1.7 times faster.
To meet the goals for 2050 would require increasing the rate of reducing f from -1.08% per year to -3.88% per year: 3.6 times faster.