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.

Our Sun is always too bright to view with the naked eye, but it is far from unchanging. It experiences cycles of magnetic activity. Areas of strong activity manifest as visible spots—sunspots—on the Sun’s surface. The year 2008, however, earned the designation as the Sun’s “blankest year” of the space age. Our Sun experienced fewer spots in 2008 than it had since the 1957 launch of Sputnik. As of March 2009, the Sun was continuing its quiet pattern.

These images from the Solar and Heliospheric Observatory (SOHO) spacecraft compare sunspots on the Sun’s surface (top row) and ultraviolet light radiating from the solar atmosphere (bottom row) at the last solar maximum (2000, left column) and at the current solar minimum (2009, right column.) The sunspot images were captured by the Michelson Doppler Imager (MDI) using filtered visible light. On March 18, 2009, the face of the Sun was spotless.

The other set of images, acquired by the Extreme Ultraviolet Imaging Telescope (EIT), shows ultraviolet light radiating from the layer of the atmosphere just above the Sun’s surface. This part of the solar atmosphere is about 60,000 Kelvin—a thousand times hotter than the surface of the Sun itself. On July 19, 2000, the solar atmosphere was pulsating with activity: in addition to several extremely bright (hot) spots around the mid-latitudes, there were also numerous prominences around the edge of the disk. On March 18, 2009, however, our star was relatively subdued.

The long stretch of minimal solar activity in 2008 and early 2009 prompted some questions about whether the Sun’s quiescence was beginning to rival that of the Maunder Minimum in the late seventeenth and early eighteenth centuries. Of the 2008 minimum, solar physicist David Hathaway of the NASA Marshall Space Flight Center says, “It’s definitely been an exceptional minimum, but only compared to the past 50 years.” Citing human observations of the Sun extending back four centuries, he continues, “If we go back 100 years, we see that the 1913 minimum was at least as long and as deep as this one.” So although the minimal activity of the Sun in 2008-2009 is exceptional for the “modern” era, it does not yet rival the lowest levels of solar activity that have ever been observed.

Centuries of observations have shown that the number of sunspots waxes and wanes over a roughly 11-year period. Sunspots exhibit other predictable behavior. If you map the location of the spots on the Sun’s surface over the course of a solar cycle, the pattern they make is shaped like a butterfly. The reason for the butterfly pattern is that the first sunspots of each new solar cycle occur mostly at the Sun’s mid-latitudes, but as the solar cycle progresses, the area of maximum sunspot production shifts toward the (solar) equator. Since regular sunspot observations began, astronomers have documented 24 cycles of sunspot activity. The images acquired in July 2000 showed the Sun near the peak of Solar Cycle 23. That cycle waned in late 2007, and Solar Cycle 24 began in early 2008, but showed minimal activity through early 2009.

The small changes in solar irradiance that occur during the solar cycle exert a small influence on Earth’s climate, with periods of intense magnetic activity (the solar maximum) producing slightly higher temperatures, and solar minimum periods such as that seen in 2008 and early 2009 likely to have the opposite effect. Periods of intense magnetic activity on the Sun can spawn severe space weather that damages infrastructure in our high-tech society.

Roughly a million miles away from our planet, the SOHO spacecraft sits between Earth and the Sun, giving us an unobstructed view of the nearest star. Besides the vernal equinox, March 20 marks annual Sun-Earth day, on which NASA celebrates daytime astronomy.

Link to article: http://earthobservatory.nasa.gov/IOTD/view.php?id=37575