Tim Radford
Warning that humans may already have emitted enough carbon dioxide to undermine the 1.5°C temperature rise threshold agreed by 195 nations last December.
T
he historic international
agreement to limit global
warming to a global average rise of 1.5°C may be a
case of too little, too late.
In December last year, 195 nations
at the Paris climate summit promised a programme of action to contain greenhouse gas emissions and
limit climate change. But UK scientists now warn that humans may
have already emitted enough carbon dioxide into the planetary atmosphere to take air temperatures
over land to above 1.5°C.
And that means nations may have
to think again about what constitutes a “safe” global temperature
threshold.
Chris Huntingford, climate modeller
at the UK’s Centre for Ecology and
Hydrology, and Lina Mercado, sen-
ior lecturer in physical geography at
the University of Exeter write in the
Scientific Reports journal that even
supposing humans stabilise greenhouse gas concentrations in the
atmosphere – and although action
has been promised, far more has to
be achieved before that could happen – temperatures over land are
very likely to go beyond the proposed limit.
Land temperatures
This is chiefly because warming lags
a decade or two behind the emissions of carbon dioxide into the
atmosphere, and because land temperatures in all climate forecasts
turn out to be higher than those
over the oceans.
For most of human history, carbon
dioxide levels in the atmosphere
have stayed below 300 parts per
million (ppm). They have now,
thanks to more than a century of
the combustion of fossil fuels and
the destruction of the forests,
passed 400 ppm, and global average temperatures are already 1°C
above the historic average.
Last year was the warmest ever, the
10 warmest years ever have all happened in this century, and for the
first half of 2016 − according to
the US space agency NASA − every
month has been warmer than any
respective month since climate scientists first began keeping systematic records in 1880.
“Even if carbon dioxide was
somehow stabilised at current
levels, additional warming will
occur”
Ominously, and almost certainly
thanks to a natural climatic phenomenon called El Niño, the average rise in global temperatures –
once again above the long term
Small island states such as the Maldives face constant devastation from climate-related hurricanes and
sea level rise. Image: United Nations University via Flickr