What do we know about our past climate, and its causes?

Summary:  Climate change has destroyed civilizations in the past, and might in the future.  Fortunately we have technology. Unfortunately that requires time to develop and implement, therefore we need science to provide forecasts.  Unfortunately climate science has become politicized.  This series of posts examines the current state of climate science.  What we know, and the theories debated by scientists on the cutting edge.

Contents of this post

1. Introduction to this series
2. A summary of what we know about our past climate (global temperatures)
3. Causes of climate change since WWII
4. What about the global cooling during the 1970’s?
5. Chapters in this series
6. For more information: other posts about climate science

(1)  Introduction to this series

Despite its importance to our civilization, climate science is grossly underfunded.  Worse, it’s become politicized.  Discussions among the public have become dominated on both sides by increasingly vehement and ignorant partisans, who treat this important science like rooting for the local football team.  Or debates among religious fanatics.

Articles on the FM website discuss climate science from two perspectives.  First, the science (fortunately running largely uncontaminated by the public chattering.  Second, the public arguments — revealing much about America’s broken observation-orientation-decision-action loop (see Wikipedia).  This series looks at the science, with some mentions of the public debate.

(2)  A summary of what we know about our past climate (global temperatures)

Earth has been warming since the early 19th century.  Warming in decade-long pulses, not at a steady rate.  It’s start before significant anthropogenic CO2 releases shows other causes at work, perhaps a combination of natural and anthropogenic causes — with the strength of anthropogenic factors increasing dramatically since WWII.

Since we only have large-scale global surface temperature records since WWII, and global satellite records since 1979, the magnitude and causes of this warming must be inferred from fragmentary record of weather proxies — both uncertain at this time (but improving).

For more information:

• How warm is the Earth? How do we measure it?, 28 January 2009
• How good are our global senses, watching our changing world?, 15 October 2010
• Looking into the past for guidance about warnings of future climate apocalypses, 17 October 2010
• Watching decadal swings in global climate (pretending to be anthropogenic global warming), 17 March 2011

(3)  Causes of climate change since WWII

“Climate change: The need to consider human forcings besides greenhouse gases“, Roger Pielke Sr et al, Eos, 10 November 2009 — Opening:

Humans are recognized as having a major role in influencing environmental variability and change, including their influence on the climate system. To advance scientists’ understanding of the role of humans within the climate system, there remains a need to resolve which of the following three hypotheses is correct:

Hypothesis 1: Human influence on climate variability and change is of minimal importance, and natural causes dominate climate variations and changes on all time scales. In coming decades, the human influence will continue to be minimal.

Hypothesis 2a: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first-order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern during the coming decades.

Hypothesis 2b: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and are dominated by the emissions into the atmosphere of greenhouse gases, the most important of which is CO2. The adverse impact of these gases on regional and global climate constitutes the primary climate issue for the coming decades.

These hypotheses are mutually exclusive. Thus, the accumulated evidence can only provide support for one of these hypotheses. The question is which one?

Hypotheses 2a and 2b are two different oppositional views to hypothesis 1. Hypotheses 2a and 2b both agree that human impacts on climate variations and changes are significant. They differ, however, with respect to which human climate forcings are important. Because hypothesis 1 is not well supported, our scientific view is that human impacts do play a significant role within the climate system. Further, we suggest that the evidence in the peer-reviewed literature (e.g., as summarized by National Research Council (NRC) [2005]) is predominantly in support of hypothesis 2a, in that a diverse range of first-order human climate forcings have been identified.

We therefore conclude that hypothesis 2a is better supported than hypothesis 2b, which is a policy that focuses on modulating carbon emissions. Hypothesis 2b as a framework to mitigate climate change will neglect the diversity of other, important first-order human climate forcings that also can have adverse effects on the climate system. … These forcings are spatially heterogeneous and include the effect

• of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008],
• the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and
• the role of changes in land use/land cover [e.g., Takata et al., 2009].

Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer. Therefore, the cost-benefit analyses regarding the mitigation of CO2 and other greenhouse gases need to be considered along with the other human climate forcings in a broader environmental context, as well as with respect to their role in the climate system.

Because hypothesis 2a is the one best supported by the evidence, policies focused on controlling the emissions of greenhouse gases must necessarily be supported by complementary policies focused on other first-order climate forcings.

The issues that society faces related to these other forcings include the increasing demands of the human population, urbanization, changes in the natural landscape and land management, long-term weather variability and change, animal and insect dynamics, industrial and vehicular emissions, and so forth. All of these issues interact with and feed back upon each other.

For more information:

• Aerosols (pollutants, like soot) as a driver of climate change, 8 May 2009
• Peer reviewed science: breakthoughs about the sun’s impact on Earth’s climate, 4 September 2009
• Might the current eruptions in Iceland become worse, affecting Europe – and perhaps our climate?, 19 May 2010
• Update about the weather – on the Sun. Perhaps coming soon to Earth., 9 February 2011

Also see this new article:  “Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks“, Geophysical Research Letters, 31 January 2012.

(4)  What about the global cooling during the 1970’s

The propaganda campaign to create hysteria about global warming required altering the public memory about both science and history.   Both have been successful, disturbingly so.

For the former, putting the well-developed science about the Little Ice Age down the memory hole.  The public must have a false image of the world’s climate as stable, so the two century warming could be attributed to late 20th century industrialization.

The latter required erasing from public memory the global cooling scare of the 1970′s.   That required amnesia about well-documented history.  For example see this typical strawman argument (refutation of an exaggerated version):  “The Myth of the 1970s Global Cooling Scientific Consensus“, Thomas C. Peterson, William M. Connolley, and John Flect, American Meteorological Society, September 2008.

The success of this project should disturb us, perhaps foreshadowing even more ambitious manipulations in the future (as the global warming hysteria built upon the “nuclear winter” propaganda).

On the FM website you’ll find some of this lost historyretrieved from the memory hole.  See these posts about global cooling:

1. An important letter sent to the President about the danger of climate change, 21 October 2009 — Scientists warning the President about global cooling
2. About those headlines from the past century about global cooling…, 2 November 2009 — they’re exaggerated.
3. A look at global warming written in a cooler and more skeptical time, giving us a better understanding of climate science, 23 November 2009
4. The facts about the 1970’s Global Cooling scare, 7 December 2009 — The CIA’s famous warning that the little ice age might return

(5)  Chapters in this series

1. What we know about our past climate, and its causes
2. Good news!  Global temperatures have stabilized, at least for now.
3. Is it possible to debate climate change with true believers? See the replies to Thursday’s post.  Comments welcomed!
4. What can climate scientists tell about the drivers of future warming?
5. What can climate scientists tell us about the drivers of future warming?  – part two of two
6. The slow solar cycle is getting a lot of attention. What are its effect on us?
7. What we’re learning about climate, and recommendations

(6)  For more information: other posts about climate science

(a)  For more information see the other FM Reference Pages:

• Science and Nature – my articles.
• Science & nature – studies & reports
• Science & Nature – general media articles
• Articles from the 1970′s about global cooling/warming

Some posts about climate change:

1. Science in action, a confused and often nasty debate among scientists, 5 February 2009
2. Lost voices in the climate science debate, 22 April 2009
3. Looking into the past for guidance about warnings of future climate apocalypses, 17 October 2010
4. Watching decadal swings in global climate (pretending to be anthropogenic global warming), 17 March 2011