As of 2018, 1.6 billion air conditioning units were installed which accounted for an estimated 20% of energy usage in buildings globally with the number expected to grow to 5.6 billion by 2050. The United Nations called for the technology to be made more sustainable to mitigate climate change using techniques including passive cooling, evaporative cooling, selective shading, windcatchers and better thermal insulation. Refrigerants used within air conditioners have caused damage to the ozone layer and are also exacerbating climate change.