NYT: Lessons from the Little Ice Age

by Geoffrey Parker, The New York Times, March 22, 2014

Columbus, Ohio — Climatologists call it the Little Ice Age; historians, the General Crisis.

During the 17th century, longer winters and cooler summers disrupted growing seasons and destroyed harvests across Europe. It was the coldest century in a period of glacial expansion that lasted from the early 14th century until the mid-19th century. The summer of 1641 was the third-coldest recorded over the past six centuries in Europe; the winter of 1641-42 was the coldest ever recorded in Scandinavia. The unusual cold that lasted from the 1620s until the 1690s included ice on both the Bosporus and the Baltic so thick that people could walk from one side to the other.

The deep cold in Europe and extreme weather events elsewhere resulted in a series of droughts, floods and harvest failures that led to forced migrations, wars and revolutions. The fatal synergy between human and natural disasters eradicated perhaps one-third of the human population.

There are two ways to consider the impact of climate change. We can predict the future based on current trends or we can study a well-documented episode of the past.

What happened in the 17th century suggests that altered weather conditions can have catastrophic political and social consequences. Today, the nation’s intelligence agencies have warned of similar repercussions as the planet warms — including more frequent but unpredictable crises involving water, food, energy supply chains and public health. States could fail, famine could overtake large populations and flood or disease could cross borders and lead to internal instability or international conflict.

Earth scientists have discerned three factors at work globally during the 17th century: increased volcanic eruptions, twice as many El Niño episodes (unusually warm ocean conditions along the tropical west coast of South America), and the virtual disappearance of sunspots, reducing solar output to warm the Earth.

The 17th century saw a proliferation of wars, civil wars and rebellions and more cases of state breakdown around the globe than any previous or subsequent age. Just in the year 1648, rebellions paralyzed both Russia (the largest state in the world) and France (the most populous state in Europe); civil wars broke out in Ukraine, England and Scotland; and irate subjects in Istanbul (Europe’s largest city) strangled Sultan Ibrahim.

Climate alone did not cause all the catastrophes of the 17th century, but it exacerbated many of them. Outbreaks of disease, especially smallpox and plague, tended to be more common when harvests were poor or failed. When an uprising by Irish Catholics on October 23, 1641, drove the Protestant minority from their homes, no one had foreseen a severe cold snap, with heavy frost and snow at a time and in a place that rarely has snow. Thousands of Protestants died of exposure, turning a political protest into a massacre that cried out for vengeance. Oliver Cromwell would later use that episode to justify his brutal campaign to restore Protestant supremacy in Ireland.

But the cold did take a more direct toll. Western Europe experienced the worst harvest of the century in 1648. Rioting broke out in Sicily, Stockholm and elsewhere when bread prices spiked. In the Alps, poor growing seasons became the norm in the 1640s, and records document the disappearance of fields, farmsteads and even whole villages as glaciers advanced to the farthest extent since the last Ice Age. One consequence of crop failures and food shortages stands out in French military records: Soldiers born in the second half of the 1600s were, on average, an inch shorter than those born after 1700, and those born in the famine years were noticeably shorter than the rest.

Few areas of the world survived the 17th century unscathed by extreme weather. In China, a combination of droughts and disastrous harvests, coupled with rising tax demands and cutbacks in government programs, unleashed a wave of banditry and chaos; starving Manchu clansmen from the north undertook a brutal conquest that lasted a generation. North America and West Africa both experienced famines and savage wars. In India, drought followed by floods killed over a million people in Gujarat between 1627 and 1630. In Japan, a mass rebellion broke out on the island of Kyushu following several poor harvests. Five years later, famine, followed by an unusually severe winter, killed perhaps 500,000 Japanese.

No human intervention can avert volcanic eruptions, halt an El Niño episode or delay the onset of drought, despite the possibility that each could cause starvation, economic dislocation and political instability. But, unlike our ancestors who faced these changes 350 years ago, today we possess both the resources and the technology to prepare for them.

Britain’s chief scientific officer has warned, for instance, that in the face of a seemingly inexorable rise in sea levels, “We must either invest more in sustainable approaches to flood and coastal management or learn to live with increased flooding.” In short, we have only two choices: pay to prepare now — or prepare to pay much more later.

The experience of Somalia provides a terrible reminder of the consequences of inaction. Drought in the region between 2010 and 2012 created local famine, exacerbated by civil war that discouraged and disrupted relief efforts and killed some 250,000 people, half of them under the age of 5.

In the 17th century, the fatal synergy of weather, wars and rebellions killed millions. A natural catastrophe of analogous proportions today — whether or not humans are to blame — could kill billions. It would also produce dislocation and violence, and compromise international security, sustainability and cooperation.

So while we procrastinate over whether human activities cause climate change, let us remember the range of climate-induced catastrophes that history shows are inevitable — and prepare accordingly.

Geoffrey Parker is a history professor at Ohio State University and the author of “Global Crisis: War, Climate Change and Catastrophe in the Seventeenth Century.”

A version of this op-ed appeared in print on March 23, 2014, on page SR3 of the New York edition with the headline: "Lessons From the Little Ice Age."

http://www.nytimes.com/2014/03/23/opinion/sunday/lessons-from-the-little-ice-age.html 

A grand solar minimum would barely make a dent in human-caused global warming

Research has shown that a grand solar minimum would offset no more than 0.3 °C of global warming

by Dana Nuccitelli, "Climate Consensus - The 97%," The Guardian, January 19, 2014

Fortunately for us, solar activity is quite stable, and a solar minimum would only have a small effect on global temperatures. Photograph: saesnes

Recent articles in the Danish newspaper Jyllands-Posten (translation available here) and in the Irish Times both ran headlines claiming that another grand solar minimum could potentially trigger an "ice age" or "mini ice age" this century. These articles actually refer to the Little Ice Age (LIA) – a period about 500 to 150 years ago when global surface temperatures were approximately 1 °C colder than they are today. This is quite different from an ice age, which are more like 5m°C colder than today. The LIA was not actually very cold on a global scale.

So, in order to trigger another LIA, a new grand solar minimum would have to cause about 1 °C cooling, plus it would have to offset the continued human-caused global warming of 1 to 5 °C by 2100, depending on how our greenhouse gas emissions change over the next century.

In the Jyllands-Posten article, Henrik Svensmark (the main scientist behind the hypothesis that the sun has a significant indirect impact on global climate via galactic cosmic rays) was a bit more measured, suggesting,

"I can imagine that it will become 0.2 °C colder. I would be surprised if it became 1–2 °C"

So these two articles are suggesting that a grand solar minimum could have a net cooling effect in the ballpark of 1 to 6 °C, depending on how human greenhouse gas emissions change over the next century. Is it plausible that a grand solar minimum could make that happen?

The short answer is, 'No.'

Fortunately, Solar Output is Stable

We're fortunate that the amount of solar radiation reaching the Earth's surface is very stable. Climate contrarians will often ask if we'd prefer if the planet were warming or cooling, suggesting that global warming is a good thing because at least the planet isn't getting colder. This is a false dichotomy - an ideal climate is a stable one.

The relatively stable climate over the past 10,000 years has allowed establishment of human civilization, by making it possible to create large stationary agricultural farms because we could rely on stable weather patterns. During that time, net global surface temperatures changes haven't exceeded 1 °C from the coldest to the hottest climates, though we're now approaching that degree of change, with 1 °C warming since the LIA, 0.8 °C of that over the past century, with much more to come.

What difference would a grand solar minimum make in the amount of solar energy reaching Earth? Two examples are the Maunder Minimum, a period of very low solar activity between 1645 and 1715, and the Dalton Minimum, a period of low (but not as low as the Maunder Minimum) solar activity between 1790 and 1830.

400 years of sunspot observations data. Created by Robert Rohde, via Wikipedia.

Relative to current levels, the Dalton Minimum represents a 0.08% decrease in the amount of solar radiation reaching the Earth's surface, and the Maunder Minimum represents a 0.25% decline. That's how stable solar activity is. That's also why we're playing with fire by increasing the greenhouse effect so much and so quickly. We're threatening the stability of the climate that has been so favorable to our development.

Peer-Reviewed Research Says Global Warming will Continue

There have been several studies in recent years investigating what impact another grand solar minimum would have on global surface temperatures, since solar research suggests it's possible we could be due for another extended solar minimum. Generally these studies will run climate model simulations under a given greenhouse gas emissions scenario with stable solar activity, then run the same scenario with the sun going into a grand minimum, and look at the difference in resulting global surface temperature changes.

Using this approach, Feulner and Rahmstorf (2010) (PDF available here) estimated that another solar minimum equivalent to the Dalton and Maunder Minima would cause 0.09 °C and 0.26 °C cooling, respectively.

The global mean temperature difference is shown for the time period 1900 to 2100 for the IPCC A2 emissions scenario. The red line shows predicted temperature change for the current level of solar activity, the blue line shows predicted temperature change for solar activity at the much lower level of the Maunder Minimum, and the black line shows observed temperatures through 2010. Adapted from Feulner & Rahmstorf (2010) by SkepticalScience.com

Jones et al. (2012) (PDF available here) arrived at a nearly identical result, with cooling from another Dalton or Maunder Minimum at 0.09 °C and 0.26 °C, respectively. Similarly, a new paper by Anet et al. (2013) found that a grand solar minimum will cause no more than 0.3 °C cooling over the 21st century.

Consistent with these previous studies, Meehl et al. (2013) (PDF available here) estimate a Maunder Minimum would cause about 0.26 °C cooling, but as soon as solar activity began to rise again, that cooling would be offset by solar warming. This is a key point, because a grand solar minimum would not be a permanent change. These solar minima last for a few decades, but eventually solar activity rises once again. Thus any cooling caused by a solar minimum would only be temporary.

The cooling effect of a grand solar minimum can also be estimated very easily without the aid of climate models, because the change in the amount of solar radiation reaching the Earth's surface is directly proportional to the temperature change it causes. Performing this calculation yields the same result as the model-based research: approximately 0.3 °C cooling from another Maunder-type grand solar minimum. Click here to see the details behind the calculation.

The Heating of the Deep Oceans

In the Jyllands-Posten article, Svensmark also disputes the data showing the accelerated accumulation of heat in the deep oceans.

"How can the ocean below 700 meters be heated up, without the upper ocean warming up accordingly?"

This is an increasingly common argument made by climate contrarians, and a bit of a strange one. The data are what they are - we've measured the deep ocean warming, including with reliable instruments on Argo buoys for close to a decade now. Even if we couldn't explain how the heat got there, it's there.

5-year averages of ocean heat content 0-700 meters (red) and 0-2000 meters (black), from the National Oceanographic Data Center

But let's address the question anyway - do we expect to have seen some obvious indication of heat being transferred from the shallow to deep ocean layers?

It's certainly not clear that we should. Consider the analogy of a bathtub. Water from the faucet represents heat entering the shallow ocean layer. Water exiting the drain represents heat leaving the shallow oceans and entering the deep oceans. The water level in the bathtub represents the heat in the shallow ocean layer (which is what we measure).

If the amount of water entering the tub from the faucet is the same as the amount of water draining out of the tub, the water level in the tub won't change. Yet the water still flows down the drain. Climate scientist Gavin Schmidt has discussed this point, summarized here.

In short, we wouldn't necessarily see the heat being transferred through the shallow to the deep oceans. However, there has been plenty of warming of the shallow oceans that could have been transferred to the deeper oceans. In our case, the water is flowing into the tub faster than it's draining out - the shallow oceans are warming fast, as the figure above illustrates.

Svensmark Gets Ocean Warming Wrong

Unfortunately Svensmark appears to be unfamiliar with this ocean heating data, saying,

"The thousands of buoys that we have deployed after 2003 to measure the ocean temperature, have not registered any temperature rise."

This is just totally wrong, even if we ignore the rapid warming of the deep oceans (as is clear from a simple examination of the figure above). The ocean heat content data can be downloaded from the National Oceanographic Data Center here. The heating trend since 2003 in the upper 700 meters of oceans is equivalent to nearly 1 Hiroshima atomic bomb detonation per second (plus another 3 per second in the deep oceans). Both the shallow and deep oceans are accumulating a whole lot of heat, with no signs of slowing whatsoever. If anything, the heating of the oceans and the planet as a whole is accelerating.

Human Influence on Climate Change is Bigger than the Sun's

The bottom line is that the sun and the amount of solar radiation reaching Earth are very stable. Even during the Maunder and Dalton grand solar minima, global cooling was relatively small - smaller than the amount of global warming caused by human greenhouse gas emissions over the past century.

A new grand solar minimum would not trigger another LIA; in fact, the maximum 0.3°C cooling would barely make a dent in the human-caused global warming over the next century. While it would be enough to offset to about a decade's worth of human-caused warming, it's also important to bear in mind that any solar cooling would only be temporary, until the end of the solar minimum.

The science is quite clear that the human influence on climate change has become bigger than the sun's. At this point, speculation about another mini ice age is pure fantasy.

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2013/aug/14/global-warming-solar-minimum-barely-dent

"Small influence of solar variability on climate over the past millennium, by A.P. Schurer, S.F.B. Tett & G.C. Hegerl, Nature Geosci. (2013); doi: 10.1038/ngeo2040

Nature Geoscience, (22 December 2013); doi: 10.1038/ngeo2040

Small influence of solar variability on climate over the past millennium

• Andrew P. Schurer,
• Simon F. B. Tett and
•  Gabriele C. Hegerl

Abstract

The climate of the past millennium was marked by substantial decadal and centennial scale variability in the Northern Hemisphere¹. Low solar activity has been linked to cooling during the Little Ice Age (AD 1450–1850; ref.  1) and there may have been solar forcing of regional warmth during the Medieval Climate Anomaly^{2, 3, 4, 5} (AD 950–1250; ref. 1). The amplitude of the associated changes is, however, poorly constrained^5, 6, with estimates of solar forcing spanning almost an order of magnitude^7, 8, 9. Numerical simulations tentatively indicate that a small amplitude best agrees with available temperature reconstructions^{10, 11, 12, 13}. Here we compare the climatic fingerprints of high and low solar forcing derived from model simulations with an ensemble of surface-air-temperature reconstructions¹⁴ for the past millennium. Our methodology¹⁵ also accounts for internal climate variability and other external drivers such as volcanic eruptions, as well as uncertainties in the proxy reconstructions and model output. We find that neither a high magnitude of solar forcing nor a strong climate effect of that forcing agree with the temperature reconstructions. We instead conclude that solar forcing probably had a minor effect on Northern Hemisphere climate over the past 1,000 years, while, volcanic eruptions and changes in greenhouse gas concentrations seem to be the most important influence over this period.

Link:  http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2040.html

"The Norse in Greenland and late Holocene sea-level change," by N. Mikkelsen, A. Kuijpers & J. Arneborg, Polar Record; doi: 10.1017/S0032247407006948

Polar Record, 44(1) (January 2008) 45–50; doi: 10.1017/S0032247407006948 

The Norse in Greenland and late Holocene sea-level change 

Naja Mikkelsen and Antoon Kuijpers (Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark) and Jette Arneborg (Greenland Research Centre at the National Museum of Denmark, Fredriksholms Kanal 12, 1220 Copenhagen, Denmark)

Abstract

Norse immigrants from Europe settled in southern Greenland in around AD 985 and managed to create a farming community during the Medieval Warm Period. The Norse vanished after approximately 500 years of existence in Greenland leaving no documentary evidence concerning why their culture foundered. The flooding of fertile grassland caused by late Holocene sea-level changes may be one of the factors that affected the Norse community. Holocene sea-level changes in Greenland are closely connected with the isostatic response of the Earth's crust to the behaviour of the Greenlandic ice sheet. An early Holocene regressive phase in south and west Greenland was reversed during the middle Holocene, and evidence is found for transgression and drowning of early-middle Holocene coast lines. This drowning started between 8 and 7 ka BP in southern Greenland and continued during the Norse era to the present. An average late Holocene sea level rise in the order of 2–3 m/1,000 years may be one of the factors that negatively affected the life of the Norse Greenlanders, and combined with other both socio-economic and environmental problems, such as increasing wind and sea ice expansion at the transition to the Little Ice Age, may eventually have led to the end of the Norse culture in Greenland.

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1439260

Extensive glaciers in northwest North America during Medieval time, by Johannes Koch & John J. Clague, Climatic Change 37 (2011)

CLIMATIC CHANGE, 37(3-4)  (2011) 593-613; DOI: 10.1007/s10584-010-0016-2

Extensive glaciers in northwest North America during Medieval time

Johannes Koch and John J. Clague

Abstract

The Medieval Warm Period is an interval of purportedly warm climate during the early part of the past millennium. The duration, areal extent, and even existence of the Medieval Warm Period have been debated; in some areas the climate of this interval appears to have been affected more by changes in precipitation than in temperature. Here, we provide new evidence showing that several glaciers in western North America advanced during Medieval time and that some glaciers achieved extents similar to those at the peak of the Little Ice Age, many hundred years later. The advances cannot be reconciled with a climate similar to that of the twentieth century, which has been argued to be an analog, and likely were the result of increased winter precipitation due to prolonged La Niña-like conditions that, in turn, may be linked to elevated solar activity. Changes in solar output may initiate a response in the tropical Pacific that directly impacts the El Niño/Southern Oscillation and associated North Pacific teleconnections.


http://www.springerlink.com/content/a42l382370422365/

What if the Sun went into a new Grand Minimum? Guest commentary by Georg Feulner, Real Climate, June 19, 2011

What if the Sun went into a new Grand Minimum?

Guest commentary by Georg Feulner, Real Climate, June 19, 2011

During a meeting of the Solar Physics Division of the American Astronomical Society, solar physicists have just announced a prediction that the Sun might enter an extended period of low activity (a ‘grand minimum’) similar to the Maunder Minimum in the 17th century. In this post I will explore the background of this announcement and discuss implications for Earth’s climate.

It has been known for a long time that solar activity shows a very regular pattern. Every 11 years the Sun is particularly active, and numerous dark sunspots are visible on its surface. These maxima of solar activity are separated by times of low activity when only few (if any) sunspots appear.

Figure 1: The Sun in visible light during an activity maximum (left) and during the last (and rather extraordinary) 11-year minimum during which it appeared spotless most of the time. Source: NASA Earth Observatory/SOHO.

One could think that the Sun emits less light during a solar maximum because of the many dark spots. In fact it is the other way round, since active regions around the sunspots emit more radiation than is “lost” in the cooler sunspot areas. This effect can be best seen in ultraviolet images of the Sun.

Figure 2: The Sun in ultraviolet light during a maximum (left) and a minimum (right). Source: NASA Earth Observatory/SOHO.

An analysis of historic sunspot observations shows that the 11-year solar activity cycle was interrupted during the late 17th century.

This period of time, during which the Sun appeared without sunspots most of the time, was called the Maunder Minimum by Jack Eddy in his famous Science paper. (Alliteratively named after Edward Maunder, although it was actually first discovered by Gustav Spörer.)

Figure 3: Observations of the number of sunspots over the last four centuries. Source: Wikimedia Commons/Global Warming Art.

The Maunder Minimum falls within the climatically cooler period of the “Little Ice Age,” during which temperatures were particularly low over continents in the Northern hemisphere (especially in winter). It has long been suspected that the low solar activity during the Maunder Minimum was one of the causes of the Little Ice Age, although other factors like a small drop in greenhouse gas concentrations around 1600 and strong volcanic eruptions during that time likely played a role as well.

Solar physicists do not yet understand how an extended solar-activity low like the Maunder Minimum arises. Yet there is recent observational evidence for an unusual behavior of the Sun during the current cycle 24, including a missing zonal wind flow within the Sun, decreasing magnetic field strength of sunspots and lower activity around the poles of the Sun. 

These observations prompted Frank Hill and colleagues to suggest that the Sun might enter a new Maunder-like minimum after the current 11-year cycle ends (i.e., after 2020 or so).

It remains to be seen whether this prognosis turns out to be true (there have been some doubts expressed), but since grand minima of solar activity did occur in the past, it is certainly interesting to explore what effects such a minimum might have on 21st century climate if it did occur. This is precisely the question Stefan Rahmstorf and I investigated in a study published last year (see also our press release. (Earlier estimates for the size of this effect can be found here and here.) In our study we find that a new Maunder Minimum would lead to a cooling of 0.3 °C in the year 2100 at most – relative to an expected anthropogenic warming of around 4 °C. (The amount of warming in the 21st century depends on assumptions about future emissions, of course).

Figure 4: Rise of global temperature (relative to 1961-1990) until the year 2100 for two different emission scenarios (A1B, red; A2, magenta). The dashed lines show the slightly reduced warming in case a Maunder-like solar minimum should occur during the 21st century. Source: PIK.

According to these results, a 21st-century Maunder Minimum would only slightly diminish future warming. Moreover, it would be only a temporary effect since all known grand solar minima have only lasted for a few decades. Critics of this result might argue that the solar forcing in these experiments is only based on the estimated change in total irradiance, which might be an underestimate, or that does not include potential indirect amplifying effects (via an ozone response to UV changes, or galactic cosmic rays affecting clouds). However, our model reproduces the historic Maunder minimum with these estimates of solar irradiance. 

Furthermore, even if one multiplied the solar effects by a huge factor of 5 (which is unrealistic), no absolute cooling would take place (the temperatures would be temporarily cooler than the base scenario, but the trends would still be warming).

It is clear that if a grand minimum were to happen it would be a tremendously exciting opportunity for solar physicists, however it is unlikely to be very exciting for anyone else.

Update 23 June: Here is a nice tongue-in-cheek video on the media response to this story.

http://www.realclimate.org/index.php/archives/2011/06/what-if-the-sun-went-into-a-new-grand-minimum/#more-7928

John Cook: A South American hockey stick

A South American hockey stick

by John Cook, Skeptical Science, September 18, 2010

A new paper has just been published employing a new technique for reconstructing past temperatures (Kellerhals 2010). It uses ammonium concentration from an ice core in tropical South America (the eastern Bolivian Andes) as a proxy for temperature. This enables them to build a temperature record going back 1600 years in a region which has had little proxy data available until now. They find a distinguishable Medieval Warm Period and Little Ice Age in the record. Nevertheless, they also find the last few decades show unprecedented warmth over the last 1600 years. 

Figure 1. Reconstructed tropical South American temperature anomalies (normalized to the 1961–1990 AD average) for the last 1600 years (red curve, smoothed with a 39‐year Gaussian filter). The shaded region envelops the ±2 standard deviation uncertainty as derived from the validation period. Poor core quality precluded any chemical analysis for the time interval between 1580 and 1640 AD.

Note that Figure 1 shows only the proxy record from the ice core -- no instrumental data is included. Of course, the usual caveat applies when looking at a single proxy record -- this is a temperature record for a single location.To get a better feel for past climate, you need to look at proxy records from a range of locations.

When we combine all the various temperature records, we find the same result: modern temperatures are significantly warmer than medieval temperatures. This is demonstrated in Moburg's reconstruction of Northern Hemisphere temperature (which happens to bear a striking resemblance to the South American proxy record). 

Figure 2. Northern Hemisphere Temperature Reconstruction by Moburg et al. (2005) shown in blue, Instrumental Northern Hemisphere Temperatures from Hadley shown in Red. Thanks to Robert Way for providing this graph.

Link:  http://www.skepticalscience.com/South-American-hockey-stick.html

The upcoming ice age has been postponed indefinitely

The upcoming ice age has been postponed indefinitely

by John Cook, Skeptical Science, January 27th, 2010

The 9th most popular skeptic argument is that we're heading into an ice age. The whole premise of the website Ice Age Now is that a new ice age could begin any day. Considering the skeptic aversion towards alarmism, it's surprising that this idea has gained so much traction. In the interest of lowering skeptics' stress levels, its time to put all those ice age fears to rest once and for all.

Just a few centuries ago, the planet experienced a mild ice age, quaintly dubbed the Little Ice Age. Part of the Little Ice Age coincided with a period of low solar activity termed the Maunder Minimum (named after astronomer Edward Maunder). It's believed that a combination of lower solar output and high volcanic activity were a major contributor (Free 1999, Crowley 2001), with changes in ocean circulation also having an effect on European temperatures (Mann 2002). 


Figure 1. Total Solar Irradiance (TSI). TSI from 1880 to 1978 from Solanki. TSI from 1979 to 2009 from PMOD.

Could we be heading into another Maunder Minimum? Solar activity is currently showing a long term cooling trend. 2009 saw solar output at its lowest level in over a century. However, predicting future solar activity is problematic. The transition from a period of 'grand maxima' (the situation in the latter 20th century) to a 'grand minima' (e.g., Maunder Minimum conditions) is a chaotic process and difficult to predict (Usoskin, 2007).
Let's say for the sake of argument that the sun does enter another Maunder Minimum over the next century. What effect would this have on Earth's climate? The difference in solar radiative forcing between Maunder Minimum levels and current solar activity is estimated between 0.17 W/m² (Wang, 2005) to 0.23 W/m² (Krivova, 2007). In contrast, the radiative forcing of CO2 since pre-industrial times is 1.66 W/m² (IPCC AR4), far outstripping solar influence. Add to this the extra CO2 emitted in upcoming decades and other greenhouse gases such as methane. The warming from man-made greenhouse gases far outstrips any potential cooling even if the sun was to return to Maunder Minimum levels.

However, our climate has experienced much more dramatic change than the Little Ice Age. Over the past 400,000 years, the planet has experienced ice age conditions, punctuated every 100,000 years or so by brief warm intervals. These warm periods, called interglacials, typically last around 10,000 years. Our current interglacial began around 11,000 years ago. Could we be on the brink of the end of our interglacial?


Figure 2. Temperature change at Vostok, Antarctica (Barnola, 2003). Interglacial periods are marked in green.

How do ice ages begin? Changes in the earth's orbit cause less sunlight (insolation) to fall on the northern hemisphere during summer. Northern ice sheets melt less during summer and gradually grow over thousands of years. This increases the Earth's albedo which amplifies the cooling, spreading the ice sheets further. This process lasts around 10,000 to 20,000 years, bringing the planet into an ice age.

Not all interglacials last the same amount of time. An ice core from Dome C, Antarctica offered a glimpse of temperatures going back 720,000 years. Climatic conditions 420,000 years ago were similar to current conditions. At that time, the interglacial lasted 28,000 years, suggesting our current interglacial may have lasted a similar period without human intervention (Augustin, 2004).

The similar conditions between now and 400,000 years ago are due to similar configurations in the Earth's orbit. At both times, the forcing from orbital variations showed much less change then in other interglacials. Simulations with the current orbit find that even without CO2 emissions, the current interglacial is expected to last at least 15,000 years (Berger, 2007).

Of course, the question of how long our interglacial lasts without human intervention is moot. We are intervening. So what effect do our CO2 emissions have on any future ice ages? This question is examined in one study that examines the glaciation "trigger" -- the required drop in summer northern insolation to begin the process of growing ice sheets (Archer 2005). The more CO2 there is in the atmosphere, the lower insolation needs to drop to trigger glaciation.

Figure 3 examines the climate response to various CO2 emission scenarios. The green line is the natural response without CO2 emissions. Blue represents an anthropogenic release of 300 gigatonnes of carbon -- we have already passed this mark. Release of 1000 gigatonnes of carbon (orange line) would prevent an ice age for 130,000 years. If anthropogenic carbon release were 5000 gigatonnes or more, glaciation will be avoided for at least half a million years. As things stand now, the combination of relatively weak orbital forcing and the long atmospheric lifetime of carbon dioxide is likely to generate a longer interglacial period than has been seen in the last 2.6 million years.


Figure 3. Effect of fossil fuel CO2 on the future evolution of global mean temperature. Green represents natural evolution, blue represents the results of anthropogenic release of 300 Gton C, orange is 1000 Gton C, and red is 5000 Gton C (Archer, 2005).
 
So we can rest assured, there is no ice age around the corner. To those with lingering doubts that an ice age might be imminent, turn your eyes towards the northern ice sheets. If they're growing, then yes, the 10,000 year process of glaciation may have begun. However, currently the Arctic permafrost is degrading, Arctic sea ice is melting and the Greenland ice sheet is losing mass at an accelerating rate. These are hardly good conditions for an imminent ice age.

Thanks to John Cross for putting me onto a few very relevant papers while preparing this post.

Link:  http://www.skepticalscience.com/upcoming-ice-age-postponed-indefinitely.html

Michael E. Mann et al., Science 326 (2009), Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly

Science (27 November 2009), Vol. 326, No. 5957, pp. 1256-1260; DOI: 10.1126/science.1177303

Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly

Michael E. Mann,^1,* Zhihua Zhang,¹ Scott Rutherford,² Raymond S. Bradley,³ Malcolm K. Hughes,⁴ Drew Shindell,⁵ Caspar Ammann,⁶ Greg Faluvegi,⁵ and Fenbiao Ni⁴ 

Abstract

Global temperatures are known to have varied over the past 1500 years, but the spatial patterns have remained poorly defined. We used a global climate proxy network to reconstruct surface temperature patterns over this interval. The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally. This period is marked by a tendency for La Niña–like conditions in the tropical Pacific. The coldest temperatures of the Little Ice Age are observed over the interval 1400 to 1700 C.E., with greatest cooling over the extratropical Northern Hemisphere continents. The patterns of temperature change imply dynamical responses of climate to natural radiative forcing changes involving El Niño and the North Atlantic Oscillation–Arctic Oscillation.

*Correspondence e-mail: mann@meteo.psu.edu

Link to abstract:  http://www.sciencemag.org/cgi/content/short/326/5957/1256

D. S. Kaufman et al., Science, 325 (2009): Recent warming reverses long-term Arctic cooling

Science (4 September 2009), Vol. 325, No. 5945, pp. 1236-1239; DOI: 10.1126/science.1173983

Reports

Recent warming reverses long-term Arctic cooling

Darrell S. Kaufman,^1,* David P. Schneider,² Nicholas P. McKay,³ Caspar M. Ammann,² Raymond S. Bradley,⁴ Keith R. Briffa,⁵ Gifford H. Miller,⁶ Bette L. Otto-Bliesner,² Jonathan T. Overpeck,³ Bo M. Vinther,⁷ and Arctic Lakes 2k Project Members

The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60° N covering the past 2,000 years, which indicates that a pervasive cooling in progress 2,000 years ago continued through the Middle Ages and into the Little Ice Age. A 2,000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

¹ School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA.
² Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80305, USA.
³ Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
⁴ Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA.
⁵ Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK.
⁶ Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA.
⁷ Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
These authors and their affiliations are presented at the end of this paper.

^*Correspondence, e-mail: darrell.kaufman@nau.edu

Link to abstract: http://www.sciencemag.org/cgi/content/abstract/325/5945/1236

M.M. Fauria et al., Climate Dynamics, Unprecedented low twentieth century winter sea ice extent in the Western Nordic Seas since A.D. 1200

Climate Dynamics (June 23, 2009); DOI: 10.1007/s00382-009-0610-z

Unprecedented low twentieth century winter sea ice extent in the Western Nordic Seas since A.D. 1200

M. Macias Fauria^{1, 2, 5, 10} , A. Grinsted^{4, 3}, S. Helama², J. Moore^{3, 6, 7}, M. Timonen⁵, T. Martma⁹, E. Isaksson⁸ and M. Eronen²

(1)	Biogeoscience Institute, University of Calgary, Calgary, AB, Canada

(2)	Department of Geology, University of Helsinki, Helsinki, Finland

(3)	Arctic Centre, University of Lapland, Rovaniemi, Finland

(4)	Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

(5)	Rovaniemi Research Station, Finnish Forest Institute, Rovaniemi, Finland

(6)	Thule Institute, University of Oulu, Oulu, Finland

(7)	College of Global Change and Earth System Science, Beijing Normal University, Beijing, China

(8)	Polar Environmental Centre, Norwegian Polar Institute, Tromsø, Norway

(9)	Institute of Geology, Tallinn University of Technology, Tallinn, Estonia

(10)	Department of Ecology, Faculty of Biology, University of Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain

(Received 1 October 2008, accepted 9 June 2009, published online 23 June 2009.)

Abstract

We reconstructed decadal to centennial variability of maximum sea ice extent in the Western Nordic Seas for A.D. 1200–1997 using a combination of a regional tree-ring chronology from the timberline area in Fennoscandia and δ¹⁸O from the Lomonosovfonna ice core in Svalbard. The reconstruction successfully explained 59% of the variance in sea ice extent based on the calibration period 1864–1997. The significance of the reconstruction statistics (reduction of error, coefficient of efficiency) is computed for the first time against a realistic noise background. The twentieth century sustained the lowest sea ice extent values since A.D. 1200: low sea ice extent also occurred before (mid-seventeenth and mid-eighteenth centuries, early fifteenth and late thirteenth centuries), but these periods were in no case as persistent as in the twentieth century. Largest sea ice extent values occurred from the seventeenth to the nineteenth centuries, during the Little Ice Age (LIA), with relatively smaller sea ice-covered area during the sixteenth century. Moderate sea ice extent occurred during thirteenth–fifteenth centuries. Reconstructed sea ice extent variability is dominated by decadal oscillations, frequently associated with decadal components of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), and multi-decadal lower frequency oscillations operating at ~50–120 year. Sea ice extent and NAO showed a non-stationary relationship during the observational period. The present low sea ice extent is unique over the last 800 years, and results from a decline started in late-nineteenth century after the LIA.

M. Macias Fauria, e-mail: mmaciasf@ucalgary.ca

Link to abstract: http://www.springerlink.com/content/922v30um17650817/