Filippo Grandi says refugees who live in a precarious situation are likely to be among the first to suffer, if the pandemic causes a global recession.
Ayse Erdogan says the Greek authorities forced her back over the border to Turkey - in contravention of international law - and she has the evidence to prove it.
President Erdogan will visit Brussels on Monday for a one-day trip, his office said, amid a row between Turkey and the EU over migrants and refugees.
Turkey has called for more help with migrants after 'opening the door' and causing increased movement on the Greek border. Meanwhile, NGOs slam the EU for what they see as a lack of humanitarian response.
More than 25 million people are forced to move every year around the world because of the effects of climate change. Experts are divided as to whether they should be granted refugee status.