Current food trade helps mitigate future climate change impacts in lower-income nations

aggregated domestic impacts (S1 Dataset): � DCðy;q;wÞ �Cðy;q;wÞ ¼ X3 i¼1 DCðy;i;q;wÞ Cðy;i;q;wÞ ∗Nðy;iÞ; and ð9Þ � DIðy;q;wÞ �Iðy;q;wÞ ¼ X3 i¼1 DIðy;i;q;wÞ Iðy;i;q;wÞ ∗Oðy;iÞ; ð10Þ Where N(y,i) and o(y,i) are supply-share weights equal to the share of each crop to the total calories consumed and imported, respectively (see S2 Fig for a graphical explanation). Climate impact on national food supply To evaluate how trade influences the climate impact on food supply for different countries (and populations), we compared their domestic production and consumption impact values. All countries with consumption impact higher than domestic production impact are classified as aggravated (i.e., climate impact on supply is heightened due to imports), and countries with consumption impact lower than domestic production impact as attenuated (i.e., climate impact on supply is lessened by imports). Countries are classified as having no effect when their domestic production and consumption impacts lie within the lowest 5th percentile of the perpendicular distance between each point and the 1:1 line of a scatter plot of the two variables (see Fig 1 top panel). Decomposition of cross-border effect To better understand what drives the difference between consumption and domestic produc- tion impact, we estimated cross-border effect (ΔC/C−ΔP*/P*), which is composed of two com- ponents (Eq 11): 1) import dependence (I/C), and 2) difference between import impact and domestic production impact (see S1 Text for the full derivation of Eq 11; S1 Dataset). DC=C DP∗=P∗ ¼ I=C∗ðDI=I DP∗=P∗Þ ð11Þ

Thus, cross-border effect is higher for countries with large important dependence and for whom climate change impacts on imports exceed those on domestic production. It is impor- tant to distinguish between cross-border and import impacts. Cross-border impact highlights the per-calorie difference between a country’s consumption and its domestic production. In contrast, import impact reflects the climate burden of imports based on the quantities brought in from other countries. If a country’s consumption heavily relies on imports, the cross-border impact might closely resemble the import impact. In such cases, the distinction between the two may blur, leading to the perception that they are the same. Results Current food trade aggravates climate impacts on food supply for higher- income countries and mitigates it for lower-income countries Under 2˚C future global mean warming and 2010 trade portfolios, both domestic production and consumption impacts (aggregated for the three crops) are negative for 56 out of 162 coun- tries (1.85 billion people; third quadrant in Fig 1, top panel). That is, climate change will reduce available calories for these countries irrespective of where they are sourced. These 56 countries constitute 49% of total global exported calories for the three crops and include large producers such as USA, Canada, and Brazil. Nevertheless, imports significantly mediate the impact of PLOS ONE Current food trade helps mitigate future climate change impacts in lower-income nations PLOS ONE | https://doi.org/10.1371/journal.pone.0314722 January 3, 2025 5 / 16