Climate change and international migration: Exploring the macroeconomic channel

2.2.2 Temperature projections. Temperature data are taken from a suite of climate model simulations conducted for the Coupled Model Intercomparison Project (CMIP) phase 6 under historical and future GHG forcing (CMIP6, [29]). Because we are interested in the effects of a substantial global warming of 3˚C, we consider two strong GHG forcing scenarios, the SSP3–7.0 and SSP5–8.5, associated with a radiative forcing of approximately 7.0 and 8.5 W/m2, respectively, by the end of the century [32]. For each model and scenario, we identify a 30-years period in which the level of 3˚C global warming is reached. To this end, we define the global warming level as DTs;mðtÞ ¼ DTðtref Þ þ �Ts;mðtÞ �Tmðtref Þ: ð1Þ where s and m define respectively the SSP scenario and the GCM. tref is a period of reference corresponding to 1986 − 2005. ΔT(tref) is the observational estimate of global warming in this period relative to pre-industrial conditions, and corresponds to 0.75˚C as reported in [33]. �Tmðtref Þ is the mean annual global temperature for the period of reference according to the cli- mate model m. �Ts;mðtÞ is the annual global mean temperature averaged for a 30-years period defined as [t − 14, t + 15], where t assumes values on 5-years intervals (i.e. at multiples of 5). The condition ΔTs,m(t) = 3˚C defines the 30-years period of interest. The periods used for each climate model and SSP scenario are reported in the S1 Table in S1 File. It is important to note that, since we are looking at a specific global warming level, the difference between the two

GHG scenarios appears only in the rate of warming, i.e. the steepness of the pathway towards the 3˚C warming level (more slowly under SSP3–7.0 than under SSP5–8.5). To account for uncertainty in the warming rate as well as in the spatial pattern of warming and internal cli- mate variability, we employ ten different GCMs. 2.2.3 Climate change effect on GDPc. Our baseline projections of future GDPc are taken from the projections produced by the OECD [21] under the five SSP narratives [20]. These narratives employ different assumptions on how our societies will evolve in the future, includ- ing changes in economic and political spheres. We focus here on SSP3 and SSP5, which are compatible with our high GHG concentration scenarios while representing rather different assumptions about future economic development. SSP3 results in relatively slow economic growth and a stagnant level of between-country inequality, as measured e.g. by the Gini-index [21]. SSP5, on the other hand, features strong economic growth and at the same time, conver- gence that strongly reduces between-country inequality. Note that while these two scenarios represent the upper and lower extremes of the SSP scenario space in terms of economic growth, the underlying economic models do not account for short-term shocks that may impede growth, and thus SSP3 may still not be the most pessimistic plausible future scenario [21]. To calculate the effect of warming on GDPc, we combine SSP3 GDPc projections with SSP3–7.0 climate projections, and SSP5 GDPc projections with SSP5–8.5 climate projections.

For each of these two baseline scenarios, we calculate two corresponding scenarios of country- level GDPc perturbed by global warming, using two alternative methods of estimating the effect of warming on the economy. The first method, named the long-term case, consists in a cross-sectional analysis of long-run weather averages, which work as proxies for climate vari- ables. This method accounts for any adaptation to climate change that is detectable in the his- torical data, and assumes that the same climate will have the same effect, despite differences in the geographical location. The second method, labelled short-term case, is based on a panel regression analysis using annual changes of temperature. While this method controls for unobserved heterogeneity, meaning that the same weather will have different effects depend- ing on the location, it does not capture adaptation strategies. The first approach employs the PLOS ONE Climate change and international migration: Exploring the macroeconomic channel PLOS ONE | https://doi.org/10.1371/journal.pone.0276764 November 16, 2022 6 / 25