Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.

Settings

Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup

Settings


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Support

Twitter Facebook YouTube Mastodon MeWe

RSS Posts RSS Comments Email Subscribe


Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...



Username
Password
New? Register here
Forgot your password?

Latest Posts

Archives

Archived Rebuttal

This is the archived Intermediate rebuttal to the climate myth "Climate's changed before". Click here to view the latest rebuttal.

What the science says...

Natural climate change in the past proves that climate is sensitive to an energy imbalance. If the planet accumulates heat, global temperatures will go up. Currently, CO2 is imposing an energy imbalance due to the enhanced greenhouse effect. Past

If there's one thing that all sides of the climate debate can agree on, it's that climate has changed naturally in the past. Long before industrial times, the planet underwent many warming and cooling periods. This has led some to conclude that if global temperatures changed naturally in the past, long before SUVs and plasma TVs, nature must be the cause of current global warming. This conclusion is the opposite of what the peer-reviewed science has found.

Our climate is governed by the following principle: when you add more heat to our climate, global temperatures rise. Conversely, when the climate loses heat, temperatures fall. Say the planet is in positive energy imbalance. More energy is coming in than radiating back out to space. This is known as radiative forcing, the change in net energy flow at the top of the atmosphere. When the Earth experiences positive radiative forcing, our climate accumulates heat and global temperature rises (not monotonically, of course, internal variability will add noise to the signal).

How much does temperature change for a given radiative forcing? This is determined by the planet's climate sensitivity. The more sensitive our climate, the greater the change in temperature. The most common way of describing climate sensitivity is the change in global temperature if atmospheric CO2 is doubled. What does this mean? The amount of energy absorbed by CO2 can be calculated using line-by-line radiative transfer codes. These results have been experimentally confirmed by satellite and surface measurements. The radiative forcing from a doubling of CO2 is 3.7 Watts per square metre (W/m2) (IPCC AR4 Section 2.3.1).

So when we talk about climate sensitivity to doubled CO2, we're talking about the change in global temperatures from a radiative forcing of 3.7 Wm-2. This forcing doesn't necessarily have to come from CO2. It can come from any factor that causes an energy imbalance.

How much does it warm if CO2 is doubled? If we lived in a climate with no feedbacks, global temperatures would rise 1.2°C (Lorius 1990). However, our climate has feedbacks, both positive and negative. The strongest positive feedback is water vapour. As temperature rises, so too does the amount of water vapour in the atmosphere. However, water vapour is a greenhouse gas which causes more warming which leads to more water vapour and so on. There are also negative feedbacks - more water vapour causes more clouds which can have both a cooling and warming effect.

What is the net feedback? Climate sensitivity can be calculated from empirical observations. One needs to find a period where we have temperature records and measurements of the various forcings that drove the climate change. Once you have the change in temperature and radiative forcing, climate sensitivity can be calculated. Figure 1 shows a summary of the peer-reviewed studies that have determined climate sensitivity from past periods (Knutti & Hegerl 2008).

Figure 1: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thick coloured bars indicate likely value (more than 66% probability). The thin coloured bars indicate most likely values (more than 90% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively.

There have been many estimates of climate sensitivity based on the instrumental record (the past 150 years). Several studies used the observed surface and ocean warming over the twentieth century and an estimate of the radiative forcing. A variety of methods have been employed - simple or intermediate-complexity models, statistical models or energy balance calculations. Satellite data for the radiation budget have also been analyzed to infer climate sensitivity.

Some recent analyses used the well-observed forcing and response to major volcanic eruptions during the twentieth century. A few studies examined palaeoclimate reconstructions from the past millennium or the period around 12,000 years ago when the planet came out of a global ice age (Last Glacial Maximum).

What can we conclude from this? We have a number of independent studies covering a range of periods, studying different aspects of climate and employing various methods of analysis. They all yield a broadly consistent range of climate sensitivity with a most likely value of 3°C for a doubling of CO2.

The combined evidence indicates that the net feedback to radiative forcing is significantly positive. There is no credible line of evidence that yields very high or very low climate sensitivity as a best estimate.

CO2 has caused an accumulation of heat in our climate. The radiative forcing from CO2 is known with high understanding and confirmed by empirical observations. The climate response to this heat build-up is determined by climate sensitivity.

Ironically, when skeptics cite past climate change, they're in fact invoking evidence for strong climate sensitivity and net positive feedback. Higher climate sensitivity means a larger climate response to CO2 forcing. Past climate change actually provides evidence that humans can affect climate now.

Updated on 2012-09-24 by John Cook.



The Consensus Project Website

THE ESCALATOR

(free to republish)


© Copyright 2024 John Cook
Home | Translations | About Us | Privacy | Contact Us