Table 3.

Present value (PV) of global social costs of CH₄ emissions, 2015–2050 (billion 2015 US$), from a harmful algal bloom sufficient to close all MI and OH beaches on Lake Erie.

	PV	PV
	Closure at 30 ppb chlorophyll aa	Closure at 50 ppb chlorophyll ab
SC-CH4 methodc
Discount rate = 5%	13.72	23.87
Discount rate = 3%	39.42	68.59
Discount rate = 2.5%	55.16	95.97
SC-CO2 × CO2-e methodd
Discount rate = 5%	9.92	17.25
Discount rate = 3%	42.12	73.28
Discount rate = 2.5%	66.08	114.96

^aA 30 ppb chlorophyll a concentration represents an 80–100% risk of Cyanobacteria blooms²¹; we associate this with a 1.003926 Tg yr⁻¹ increase in CH₄ emissions.

^bThe World Health Organization chlorophyll a guideline for avoiding moderate probability of adverse health effects in recreational waters is 50 ppb²²; our estimates suggest an associated increase of 1.746587 Tg yr⁻¹ in CH₄ emissions.

^cSC-CH₄ method uses estimates of the social costs of CO₂, CH₄, and N₂O adapted from published sources^16,29.

^dSC-CO₂ × CO₂-e method converts CH₄ to CO₂-equivalents and uses estimates of the social cost of carbon dioxide¹⁵.