Illustration: Sarah Grillo/Axios

A new study concludes that rising temperatures have trimmed rural-to-urban migration within very poor nations while slightly increasing it in middle-income countries.

Why it matters: The analysis, released via the National Bureau of Economic Research, provides a deeper understanding about how warming has already begun affecting human movement — and will in the future.

The impact: "We project that expected warming in the next century will encourage further urbanization in middle-income countries such as Argentina, but it will slow down urban transition in poor countries like Malawi and Niger."

What they did: Researchers with UC Davis and the University of Idaho looked at migration data for huge numbers of relatively small geographic "cells" worldwide from 1970 to 2000.

  • They combined it with other data on temperatures, population and precipitation in the same regions.
  • They broke up countries into 3 wide groups: nations in the bottom 25% of per capita GDP, the middle 50%, and the top 25%.

What they found: The middle-income group showed the most internal migration to cities. A big takeaway: Declines in rural productivity, including lower crop yields stemming from climate change, have different effects on poor vs. middle-income nations.

  • Why? They see a vicious cycle in very poor places: Hotter temperatures cut rural productivity, making migration costs even less feasible.
  • "On the other hand, rising temperatures increase out-migration from rural areas in middle-income countries because temperature shocks widen the rural-urban income gaps, which work to strengthen individuals’ incentives to migrate, once they can pay for migration costs," the study states.

Go deeper: Youth protests sweep the globe demanding action on climate change

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