Some parts of nature and human society are more vulnerable than expected to climate change, according to a draft of a UN report that adds a new purple colour to a key diagram to show worsening risks beyond the red used so far.

It says “unique and threatened systems” like coral reefs, endangered animals and plants, Arctic indigenous communities, tropical glaciers or small island states seem to be less able to adapt to warming than believed in a last report in 2007.

The 44-page draft Summary for Policymakers by the Intergovernmental Panel on Climate Change (IPCC) looking at impacts of climate change worldwide, obtained by Reuters and dated March 2013, is part of a series of IPCC reports updating science from 2007 as the main guide for governments.

It will be issued in March 2014 in Japan after several rounds of editing by experts. “It would be misleading to draw conclusions from it,” said Jonathan Lynn, spokesman for the IPCC Secretariat.

Still, the draft by the world’s leading experts introduces purple to the diagram summarising risks, often called the “burning embers” since it shows vertical bars that turn red towards the top on a scale of average world temperatures rising up to an extreme of 5˚C.

The base of the five bars is white or yellow, showing lower risks, but shifts to red as temperatures rise.

“Purple colour, introduced here for the first time, reflects the assessment that unique human and natural systems tend to have very limited adaptive capacity” to rising temperatures, the draft says. And many are facing multiple threats.

The new purple replaces red at about a 2˚C rise over current levels to indicate extreme risks to many unique and threatened human and natural systems.

Recent scientific findings show that coral reefs, for instance, are vulnerable to death because of warming and the less well understood impact of acidification of the oceans, both linked to a build-up of carbon dioxide in the air.

And “hot spots” of diversity of endangered animals and plants may be at greater risk, partly because climate change adds to stresses such as loss of habitats, hunting and invasive species.