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Judith Curry: How Urgent Is ‘Urgent’?

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Judith Curry, Climate Etc

The next 10 years (or even 5 years) will provide substantial clarity on the relative importance of human-caused and naturally varying climate change, and how rapidly humans can be expected to change the climate in the 21st century.

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I think we have a very brief window of opportunity to deal with climate change . . . no longer than a decade at most. – James Hansen 2006

We have only four more years to act on climate change. – James Hansen 2009

It looks like our time is already up, or maybe we have two more years, depending on whether you work from the 2006 or 2009 version.

If you are working from the 2C ‘dangerous’ target, Michael Mann argued recently that Earth will cross the danger threshold in 2036.

Now, today we hear from the Sunday TimesJust 16 years to avoid carbon calamity, say experts

AR5 Synthesis Report

This particular post is triggered by today’s release of the IPCC AR5 Synthesis Report and Press Release and Press Conference.  A good summary of what has been going on is given by this BBC article Fossil fuels should be phased out by 2100 says IPCC.  The highlights:

* CO2 emissions must be reducedby almost half by 2030 or global temperaures will eventually rise by between 2C and 5C.

* Humans must pump no more than a further one trillion tonnes of CO2 into the atmosphere if temperature change is to be kept below 2C

* To keep warming below 2°C, the world will have to cut greenhouse gas emissions between 40 and 70 percent by 2050—and then keep cutting until they’re essentially zero by 2100.

Tweeted comments from Ban-Ki Moon’s press conference:

* “Science has spoken. There is no ambiguity in their message. Leaders must act. Time is not on our side.”

* “When your child is sick with a high temperature, you have to take all the medicine”

* “ Synthesis Report gives major push for Paris, mobilise action to 2C pathway”

How long do we have to act?

Lets accept for the sake of argument that there is a risk that adding CO2 will eventually cause undesirable climate change.  Further, there seems to be broad agreement that it is in everyone’s long term interests to move away from fossil fuels as a primary energy source (these resources are finite, at some point they will become very costly to extract, and there are pollution/health issues associated with burning fossil fuels).

But how urgently do we need to act in terms decarbonization, even if you buy the 2C danger limit?  The 16 year deadline comes from the business as usual emissions scenario, whereby climate model projections state that the 2C threshold would likely be crossed in 2040.

Here is why it is increasingly unlikely that that we will reach the 2C danger limit by 2040:

* the ongoing surface temperature hiatus, which may continue until the 2030’s or even 2040 if the increasing number of hypotheses about AMO, PDO and natural internal variability are correct.

* the growing number of observation-based climate sensitivity studies that find lower values of transient and equilibrium climate sensitivity (e.g Lewis & Curry, WSJ op-ed).

* unrealistic scenarios of future coal burning by the IPCC (see Dave Rutledge’s previous posts)

* underestimate by 16% of plant CO2 absorption [link]

So how much do these factors individually and collectively delay the warming, beyond 2040?   Well, the hiatus one is pretty straightforward.   It has been estimated that Lewis and Curry TCR estimate delays the warming by 10 yrs.  No estimate that I ‘ve seen re delays associated with carbon budget scenarios.

What does 10 years buy us?

For the sake of argument, lets play it conservative and assume that these factors buy us 10 more years (personally, I think much longer), beyond the IPCC’s time scale.  What difference does 10 years make?

Lets look back 10 years ago, to 2004, or even to 2006 when Hansen made his first proclamation:

* fracking wasn’t on the radar screen

* there was very little penetration of wind and solar power

* there was optimism about cap and trade policies

* the pause was less than 10 years, and not yet identified as such

* the U.S. was the leader in CO2 emissions

* the massive Chinese modernization was just underway

* devastating hurricane landfalls in the US in 2004/2005

* others?

Things look pretty different now than they did 10 years ago.

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