Each year the heat gets more extreme in Tucson, Arizona. By 9.00 am Jennifer Cárdenas’s children can no longer play out in the yard, and to visit her mother‑in‑law — a short walk she could once make — she now has to drive. When their home air conditioning breaks down, temperatures reach such high and dangerous levels that they have to leave. Keeping the three‑bedroom house cool costs her hundreds of dollars in electricity. The last bill was $450…