Nclimate 2310-f2

Scientists know greenhouse gases are causing the
world to warm. But an interesting question is why warming at
earth’s surface speeds up and slows down.

A
new paper
shows surface temperature
“slowdowns” like we’re experiencing now aren’t unusual – and
capturing the timing of natural ups and down in the climate is key
to predicting them.

But as a
second paper
explains, the planet as a whole
has warmed up in the last decade even as surface temperature rise
has been sluggish.

Model mismatch

Temperatures are rising due to long term
greenhouse gas warming. But natural variability causes temperatures
to go up and down from one year to the next.

Natural variability can at least partly explain
slower surface warming in the last 15 years, the Intergovernmental
Panel on Climate Change (IPCC)
concluded
in its latest report. Recent
evidence
points
to changes in the
Pacific
causing the
deep oceans
to absorb more heat.

But most climate models didn’t predict the
slowdown. And as a
new paper
in Nature Climate Change explains,
some parts of the media have argued that since models don’t
replicate recent temperatures, we shouldn’t trust their predictions
for future warming.

But the paper, lead by Australian climate
scientist Dr James Risbey, finds that 15 years of temperatures
rising slower than models predict “does not constitute evidence
against the fidelity” of models in general. Let’s take a closer
look at why not.

It’s not unusual

Scientists predict how temperatures will change
several decades into the future by combining the results from
lots of individual model simulations. And it works pretty well –
long term projections from models agree well with what we’ve
seen.

But over short time periods, like 10 or 15
years. temperatures are influenced by the ups and downs of natural
variability. Right now, surface temperatures are at the lower end
of where climate models estimate they should be.

At other times, temperatures have been higher
than models predicted. The graph below shows that neither is
unusual.

The grey shading is the range of model
projections. The blue and red dashed lines are the rate of warming
over consecutive 15-year periods for two global temperature
datasets (each marker is the midpoint of one 15-year
trend.)

The coloured lines show the rate of global average
surface temperature rise calculated for successive 15-year periods,
plotted at the central year. Grey shading is the range of IPCC
model projections. Source: Risbey et al., (
2014
)

There are several periods when real world
temperatures are at the warm end of the models (see 1925, 1935,
1955). But in 1890, 1905, 1945, 1970 and 2005 the trend is towards
the colder end. As the paper puts it:

“In other words, the
recent ‘hiatus’ centred about 2005 (1998-2012) is not exceptional
in context … One expects the observed trend … to bounce about
within the model trend envelope in response to variations in …
ocean heat uptake rates, as they do.”

A question of timing

Climate models include all the natural
variability scientists know about but they’re not designed to
predict exact timing of when we’ll see warming speed up or slow
down, say the authors.

Here’s where the paper comes up with an
interesting idea.

The authors assume that out of all of the model
simulations, some will have got the timing of natural variability
ups and downs almost exactly right – by chance. By looking at only
these models, they found they could accurately forecast the
occurrence and timing of the most recent surface warming
slowdown.

Understanding natural variability is crucial to
predicting how temperatures will change 10 or 15 years into
the future. And these results suggests climate models can do a good
job on those timescales – as well as longer ones – provided they
capture natural variability well enough.

This won’t come as a surprise to a lot of other
scientists working in this area, however. The recent IPCC
report
did a similar test in its recent
report, coming to the same conclusion.

Big picture

It’s tempting to just look at surface
temperatures to see how the planet is changing, because it’s where
humans live. But little over two per cent of the heat reaching
earth stays in the atmosphere. 93 per cent goes into oceans, with
the rest shared between land and ice.

A
separate paper
in the journal Geophysical
Research Letters shows that despite the recent period of slower
surface warming, the climate system as a whole has continued to
warm.

Using the latest satellite measurements of how
much energy enters earth’s atmosphere compared to how much leaves,
professor Richard Allan from the University of Reading and
colleagues show earth absorbed more heat between 2000 to 2012 than
1985 to 1999.

The world as a whole is warming, and will keep
warming as long as greenhouse gases keep rising. While models do a
good job of anticipating long term warming, this new research looks
like a step towards forecasting the ups and downs we can expect
along the way.

Via: http://www.carbonbrief.org/blog/2014/07/slow-surface-warming-since-1998-is-not-exceptional-say-scientists/