Dirty Clouds

In my previous post I gave a short summary of how clouds affect the radiation and heat budget of the Earth, and the difference between low and high clouds. In a world with a changing climate, we all want to know what the effects of CO2, aerosols, and associated global warming will be on clouds and their feedback on the system. 

In truth, clouds are difficult to incorporate in climate models. They are small, the physics behind them are complex and not always well understood, and clouds have strong regional effects, all of which makes them complicated to model. Even so, knowledge of clouds is greatly improving, and so models are becoming more certain (for those interested in climate model certainty, always a hot topic, please see NASA on climate uncertainties and the IPCC on the confidence of model predictions in the 4th Assessment Report). 

First, let's just do a quick recap on what aerosols are. Basically, aerosols are all sorts of types of suspended particulate matter. Some occur natural, such as dust or organic compounds. A famous example of the latter can be found in the Blue Mountains in New South Wales, Australia, where the eucalyptus trees release a sort of organic matter into the mountain air. Incoming bluish ultraviolet light is scattered by the particles, giving the Blue Mountains their etymological characteristic.

Blue Mountains, NSW, Australia

Source: National Geographic

However, humans are putting in a fair share of aerosols into the atmosphere as well. In a previous post I discussed the effect of soot deposition on snow in Antarctica. Other aerosols include sulphates from fossil fuels and other pollutants. In the picture below, Ramanathan and Feng (2009) have indicated the emissions of black carbon across the globe.

Emission of BC in tons per year across the globe. Note the emissions across the ocean along traffic routes of airplanes and ships. 

Source: Ramanathan & Feng 2009.

Aerosols alone can affect the incoming solar radiation by reflecting some of it back into space. In this sense, they increase the albedo. However, some of the radiation is absorbed, warming the atmosphere. As a net effect, the surface is cooled, and the atmosphere warmed (Ramanathan & Carmichael 2008).

In any case, like I said above, I want to investigate the effect of aerosols on clouds. Clouds are complicated, and studying the relation between clouds and aerosols is still not well understood. The IPCC rates the scientific understanding of the interactions generally as low to very low (also see the relevant section here). Even so, I want to discuss some of the basic processes.

Aerosols affecting cloud behaviour. 

Source: UK Met Office

Clouds form around small particles, called Cloud Condensation Nuclei (CCN). More aerosols mean more CCN, meaning more droplets that together form clouds. It also means that clouds become whiter, and thus more reflective. In this sense, aerosols can decrease cloud albedo and have a cooling effect (this is the first effect in the figure above).

Because there are more CCN, the droplets are smaller and it will take longer for clouds to form raindrops. This increases the lifetime of clouds as well as decreasing the number of rainfall events. This secondary effect, also known as the Albrecht effect, is not very well understood.

Lastly, there is the semi-direct effect. The aerosols in the clouds absorb radiation, and re-emit it, warming the clouds and reducing the upward flow of moisture - and the formation of clouds. This is perhaps the least-well understood effect.

I hope that this shows how complicated the relationship between clouds and aerosols is; there is no clear-cut correlation. Some effects increase cloud cover, whereas others reduce them. Clouds are difficult to study and model, which makes it tricky to incorporate them in climate projections.

Despite all this, scientists discover more and more about aerosols and clouds, which greatly improves the models for future climate projections. To finish off, I found this animated video of the cloud-aerosol effects I explained in this post.

Another interesting field related to clouds and aerosols has to do with geo-engineering, something I want to look at in my next blog post. For more information on research that is being done on clouds, I can recommend the Guardian article in the "Further Reading" section below, along with some other interesting websites. Happy browsing!

Further Reading

• "Creating clouds in the lab to better understand climate" via The Guardian. 
• "Climate science: The aerosol effect" - a commentary in Nature on progress in the cloud-aerosol research. 
• "What is global dimming?" - an explanation of air pollution and radiation at the Earth's surface, via The Guardian. 

References

• Ramanathan V. and G. Carmichael (2008), Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221, doi:10.1038/ngeo156
• Ramanathan, V., and Y. Feng (2009), Air pollution, greenhouse gases and climate change: Global and regional perspectives, Atmospheric Environment, 43, 37, doi:10.1016/j.atmosenv.2008.09.063

Every Cloud Has a Silver Lining: Albedo & Clouds

One of the most important components of the weather system, clouds, actually has a very important effect on the world's albedo. A lot of research is being done on different types of clouds - darker clouds have a lower albedo than white ones - how clouds are formed, and what the effects of global warming will be.

Night clouds 

(Source: National Geographic)

Generally, clouds have a higher albedo than the surface underneath them, so they reflect more incoming sunlight than if they would be absent. In this sense, they have a cooling effect on the climate system. More clouds, more sunlight reflected, more cooling - or not?

The radiation coming from the sun is mostly in the same wavelength order as visual light, and is called shortwave radiation. The albedo of a given surface is the fraction of shortwave light that is being refracted. Nevertheless, not all of the light is reflected (as you'll remember from the first post). Some of the solar radiation is absorbed by the Earth, warming up the planet in the same way you get warmer from sitting in the sun. The Earth emits some of that energy in the form of longwave radiation, or heat.

In this image, you can see how clouds affect the distribution of shortwave and longwave radiation (Source: NASA)

Low and high clouds have different effects on the radiation budget. High clouds are usually thin and have low albedos. All things emit heat, even clouds, but because high clouds are quite cold, they emit less heat back into space. These two effects combined mean that high clouds have a net warming effect (also see NASA).

Low clouds do the opposite. They are thicker and have a higher albedo, and send more incoming shortwave radiation back to space before it can even reach the Earth's surface. They are closer to the Earth's surface, and their temperature is almost the same. This means that they radiate almost the same amount of longwave radiation as the planet. Even though they trap a great deal of longwave radiation as well, their net effect is cooling.

The image below is a compilation of cloud fraction cover as observed by NASA satellites. I recommend checking out the links below for further reading; these are also the links I used to do the research for this entry. Next entry I will look at the effect of pollution and aerosols on clouds and climate.

This animation shows the cloud fraction from 2005 to 2012

(Source: NASA Earth Observations)

Further Reading & References

• NASA's Earth Observatory - On clouds, cloud albedo and the effect on our climate.
• IPCC's Fourth Assessment Report - On the cloud-albedo feedback
• NOAA (National Oceanic and Atmospheric Administration) - On clouds

From Icehouse to Hothouse: Breaking Out of Global Glaciation

Last week's entry was about how a global onset of glaciation could take place, the so-called runaway albedo feedback. Quick recap: due to some sort of cooling event, ice sheets start forming on the poles and beyond, increasing the Earth's albedo. The vast expanses of ice reflect high amounts of the incoming sunlight back into space, cooling the surface even further, and a runaway effect takes place, covering the Earth completely in ice, even the tropics. These "Snowball Earth" events have taken place in the Cryogenian,  800-550 million years ago. Recent research suggests that the Earth may have actually been ice-free in some spots near the equator, a hypothesis promptly named "Slushball Earth".

What has puzzled scientists is not only how the Earth might have wound up covered in ice, but also how these icy conditions were reversed (which obviously happened at some point, considering that there are not meters of ice covering me right now as I'm sitting behind my desk in London). It would have been very difficult for a world with such a high albedo to warm up again.

Imagine a world covered in ice, like this icy plain in the Arctic 

(Source: Reuters)

CO2, carbon dioxide, is a greenhouse gas, which traps part of the outgoing radiation of the Earth. The higher the concentration in the atmosphere, the more heat from will be trapped (creating the well-known greenhouse effect). The original proponents of the Snowball Earth hypothesis, Hoffman et al. (1998), suggested that extreme amounts of CO2 could have defrosted the planet.

The mechanism is relatively simple. While the Earth is in a state of glaciation, vulcanoes continue to work, sticking out of the ice like pimples. Vulcanoes are notorious emitters of CO2. The carbon dioxide they emit builds up in the atmosphere during the millions of years of glaciation. At some point, there is so much CO2 in the atmosphere that temperatures start to rise and the ice begins to melt. This reduces the albedo, which means more solar heat is absorbed by the now open ocean waters and land masses, leading to higher temperatures, leading to more ice melt, reducing the albedo... Etc.

This video shows how vulcanoes might have broken through the snowball state of the planet. I do want to add as a side note that this is a rather dated video and so it does not take some of the "Slushball" hypotheses into account, nor does it examine the arguments of opponents of "Snowball Earth" theory (also see last week's post). Having said that, it does explain nicely in the first seven minutes what the relationship between vulcanoes, CO2 and deglaciation is, which is why I've added it to this post.

This feedback loop would have taken place very rapidly (on a geological timescale, that is). In a mere couple of millions of years, the Earth would have bounced from a complete icehouse to a hothouse (Hoffman et al. 1998).

To return the planet from a "Snowball" or "Slushball" state, a lot of CO2 is needed (Pierrehumbert 2004). Crowley et al. (2001) calculated that about 120,000 ppm (parts per million) of CO2 would be required - which is roughly 300 times of the amount of CO2 currently in the air.

In a research conducted by Bao et al. (2009), oxygen isotopes in sulphate deposits in Svalbard from 635 Million years ago, roughly the end of the Marinoan glaciation, and one of the supposed "Snowball Earth" events, were studied to find whether CO2 levels had been that high. They concluded that either the oxygen cycle must have been very, very different from what it is today, or CO2 levels were extremely high, supporting the case for a "Snowball Earth".

Eyjafjallajökull, an Icelandic vulcano, erupts in 2010 

(Source: National Geographic)

Nevertheless, that was not the end of the matter. An even more recent study by Sansjofre et al. (2011) reinterpreted the data, and found that CO2 levels were more likely around 3200 ppm, quite a bit less than the required 120,000 ppm. They suggest that glaciation was not as intense as thought before, or that perhaps there were different glaciation mechanisms at play.

To conclude; there is evidence that the world was once subject to intense glaciation, the extent of which remains under debate. Moreover, the end of this period and the processes behind the defrost are still uncertain and continue to be researched. Even so, it is interesting to see how important albedo can be, providing a feedback mechanism to enable glaciation, and to reverse it.

I wanted to add these two posts about the "Snowball Earth" hypothesis to give a bit of a framework when it comes to ice cover in the extremest sense of the word, even though "Snowball Earth" took place more than 550 million years ago. It is by studying these obscure details of the planet's natural history that we are able to better understand climate in general, and what that means for us today.

Next post: clouds - perhaps the most enigmatic (and important) component of the climate system.

References

• Bao, H., I.J. Fairchild, P.M. Wynn, and C. Spötl (2009), Surface environments: Neoproterozoic glacial lakes from Svalbard, Science, 323, 119, doi:10.1126/science.1165373
• Crowley, T.J., W.T. Hyde, and W.R. Peltier (2001), CO2 levels required for deglaciation of a "Near-Snowball" Earth, Geophys. Res. Let., 28, 283
• Hoffman, P.F., A.J. Kaufman, G.P. Halverson, and D.P. Schrag (1998), A Neoproterozoic Snowball Earth, Science, 281, 1342, doi:10.1126/science.281.5381.1342
• Pierrehumbert, R.T. (2004), High levels of atmospheric carbon dioxide necessary for the termination of global glaciation, Nature, 419, 646, doi:10.1038/nature02640
• Sansjofre, P., M. Ader, R.I.F. Trindade, M. Elie, J. Lyons, P. Cartigny, and A.C.R. Nogueira (2011), A carbon isotope challenge to the snowball Earth, Nature, 478, 93, doi:10.1038/nature10499

Frozen Planet: Snow or Slush?

So far, we've mainly looked at the present day climate mechanisms, but for this blog post, I want to go back in time - and quite a bit. Between 800-550 million years ago, during the period in the Neoproterozoic that that geologists call the Cryogenian (from Greek cryos, "cold" and genesis, "birth"), some scientists believe that the Earth was covered largely by ice, in some places up to 5 km of it. This "Snowball Earth" hypothesis  by Hoffman et al. (1998) has been scrutinised, critisised (Allen and Etienne 2008) and praised - but how does it actually work?

There are several possible situations that can trigger a Snowball Earth, but we'll examine those later. For now, let's just assume it gets cold. Really, really cold. Ice starts creeping from the poles to lower latitudes, a bit like the more recent Ice Ages. But unlike those, they don't stop. The ice goes further towards the equator. Meanwhile, all that icy surface reflects a lot of sunlight back into space because of it's high albedo, causing the planet to cool even further. At some point, there is so much ice that a runaway effect takes place: the planet's albedo is so high, that the amount of sunlight that is absorbed is too small to keep the Earth warm, and the planet freezes over.

Geologists believe that this feedback scenario happened during the Cryogenian, and that it happen not just once; most likely, three times. The BBC has made an informative short film that explains how scientists found evidence for a Snowball Earth event.

There is much uncertainty as what triggered a Snowball Earth. It has been suggested that the evolution of photosynthetic organisms caused a reduction in methane (a potent greenhouse gas), as the oxygen they put into the atmosphere reacted with the methane to form the weaker greenhouse gas, CO2. This could have ultimately lead to a cooling effect (Kopp et al. 2005).

Another theory proposes that the break-up of the continent Rodinia as the onset of the cooling (Donnadieu et al. 2004). The researchers looked at increased runoff following the continental disintegration. Higher runoff would mean more weathering, which through chemical processes could lead to a reduction in CO2 and thus to cooling.

Reconstruction of the breakup of Rodinia, 750 million years ago, based on palaeomagnetic data (Torsvik 2003)

Despite these and other proposed mechanisms (lower solar input, changes in orbital forcing, etc., also see Hoffman et al. 1998), there has been no conclusive answer as how the world changed into an ice house.

Dropstones in formations in the Flinders Ranges, SA, Australia

Some scientists have further studied the dropstones, and in fact discovered evidence that seems to contradict a world fully covered by ice during some of the glaciations (Le Heron et al. 2011). Markings on the dropstones from the Flinders Ranges shows that they were deposited by ice, but have been affected by turbulent waters as well. If ice had covered all the oceans, the water underneath it would have been gentle and calm. The turbulence would have been caused by storms raging over ice free patches; instead of Snowball Earth, there would have been a "Slushball" Earth. They suggest that these patches are the places in which some organisms managed to survive the chilly conditions (Allen & Etienne 2008, Le Heron et al. 2011), and ultimately emerge when the ice disappeared again.

Artist's impression of "Slushball" Earth, with pockets of ocean

For the next blog entry, I'll look at how the planet came out of this icehouse situation - not going into a greenhouse, but a hothouse, and how that too was impacted by the albedo effect.

Further Reading

• Snowballearth.org, a website maintained by Hoffman and Schrag, pro-Snowball scientists
• 'Life may have survived 'Snowball Earth' in ocean pockets', BBC article on the work of Le Heron et al. 2011
• 'Snowball Earth was more like a slushball', Discovery News article

References

• Allen, P.A., and J.L. Etienne (2008), Sedimentary challenge to Snowball Earth, Nature Geoscience, 1, 817, doi:10.1038/ngeo355
• Donnadieu, Y., Y. Goddéris, G. Ramstein, A. Nédélec, and J. Meert (2004), A 'snowball Earth' climate triggered by continental break-up through changes in runoff, Nature, 428, 303, doi:10.1038/nature02408
• Hoffman, P.F., A.J. Kaufman, G.P. Halverson, and D.P. Schrag (1998), A Neoproterozoic Snowball Earth, Science, 281, 1342, doi:10.1126/science.281.5381.1342
• Kopp, R.E., J.L. Kirschvink, I.A. Hilburn, and C.Z. Nash (2005), The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis, PNAS, 102, 11131, doi:10.1073/pnas.0504878102
• Le Heron, D.P., G. Cox, A. Trundley, and A. Collins (2011), Sea ice-free conditions during the Sturtian glaciation (early Cryogenian), South Australia, Geology, 39, 31, doi:10.1130/G31547.1
• Torsvik, T.H. (2003), The Rodinia jigsaw puzzle, Science, 300, 1379, doi:10.1126/science.1083469

Antarctica's Bright Future

The Antarctic ice sheets are extensive; stretching out for 14 million square km, they contain nearly 90% of all the freshwater on the planet. Melting all of the ice would lead to an increase in sea level of 60 m (NSIDC). Taking into account that Antarctica is one of the coldest places on Earth with a balmy average summer temperature of -20 ºC and a slightly cooler winter temperature of -60 ºC, this seems unlikely to happen. But then there was global warming — so should we get worried and start moving inland?

Contrary to the Arctic, which is a sea surrounded by land, Antarctica is land surrounded by sea. This means that precipitation plays an important role in ice formation, which in the Arctic ocean is less so, because most precipitation occurs on or near land. Interestingly, this snow could also mean that Antarctica might stay cool despite warming from climate change.

One of the effects of global warming is that precipitation over the Antarctic ice sheets is projected to increase (also see the 4th Assessment Report of the IPCC on Antarctica). A recent study done by Picard et al. (2012) looked at the importance of fresh snow on the albedo. It turns out that it is in fact very important.

Fresh, pristinely, white snow reflects about 85% of the sunlight. As I wrote in my previous blog post, this means that any pollution that makes the snow greyer reduces the albedo, so the snow will trap more heat and is thus more susceptible to melt. Soot can have this effect. It is stronger in the Arctic than on Antarctica, because most of the sources of pollution are on the Northern Hemisphere.

Snow is sharply faceted when it is fresh and becomes rounder and larger as it ages. The bigger the crystals, the darker the snow becomes. This is illustrated in the picture below, which is taken with an electron microscope.

Left: Fresh snow is sharper and more reflective. 

Right: As snow ages, it becomes smoother and larger, and less reflective. 

(Source: Electron and Confocal Microscopy Laboratory, USDA via NOAA)

Old snow thus reduces the albedo. In the study conducted by Picard et al., satellite data and model outputs were used to see how strong this effect is, and whether the projected increase in precipitation would lower the albedo of the Antarctic ice sheets.

They found that there is a very consistent cycle of snow grain size. In winter, the crystals are small and therefore bright. When it becomes warmer in the Antarctic summer (December in the Southern Hemisphere) the grains start to grow. In summers with high precipitation, the growing of the grain sizes is less, and the ice stays lighter, leading to increases in albedo of about 0.02-0.03 compared to normal summers. This could lead to a drop in surface temperature of 0.5 ºC in summer, and 0.3 ºC on a yearly average. Moreover, the additional snow could offset some of the loss in ice.

Picard et al. point out that there is also much research that still needs to be done when it comes to the interaction between snow and the climate system. Many climate models do not incorporate (all of) the characteristics of snow and its behaviour. This study shows that something seemingly small such as snow grain size actually plays an important role in albedo, and thus in the climate.

It is interesting to compare these results with studies done on Greenland, which has the largest land ice sheets in the Northern hemisphere. Contrary to Antarctica, Greenland's ice has been shown to darken in the last years (also see the Arctic Report by the NOAA). A recent analysis by Box et al. (2012) identified three reasons for this. First of all, there have been more warm air currents over the ice sheets that have increased the snow grain growth, so more of the snow crystals have become large and smooth, which has reduced the albedo. Secondly, more solar heat has reached the surface, warming it, and decreased the albedo through that process. Thirdly, there has been less snowfall, so there has been little fresh snow to raise the albedo (contrary to what is happening in Antarctica).

Combined, these processes have started to reduce the albedo in Greenland quite severely. In the picture below, the albedo over Greenland has been indicated, based on satellite data. In particular near the edges of the ice sheet, the albedo has dwindled strongly. This has brought up concerns as to the melting of the Greenland ice sheet, which like that of Antarctica, stores a large amount of freshwater and could raise sea levels by 6 metres (NSIDC).

The changes in albedo over the Greenland ice sheet in the summer of 2011 compared to the average of 2000-2006. The darker the area is, the greater the reduction in albedo. This is a combination of increase in snow grain size, surface melting and soot. 

(Source: NOAA)

To conclude, where Antarctic ice sheets are maintaining and/or increasing their albedo, the ice sheets of Greenland are only becoming greyer and lower in albedo. In this case, we might literally say that Greenland's future is a lot less bright than that of Antarctica.

Further Reading

• Arctic vs Antarctic (NSIDC) on the differences between the North and South Pole in terms of sea ice
• About Antarctica, by the British Antarctic Survey, on the characteristics of Antarctica
• Highlights of the Arctic Report Card, by the National Oceanic and Atmospheric Administration (NOAA), on the darkening of Greenland
• Snowfall Brightens Antarctic Future, an analysis of the research of Picard et al. by Charles Zender for Nature Climate Change

References

• Box, J.E., Fettweis, X., Stroeve, J.C., Tedesco, M., Hall, D.K., and K. Steffen (2012), Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821, doi:10.5194/tc-6-821-2012
• Picard, G., Domine, F., Krinner, G., Arnaud, L., and E. Lefebvre (2012), Inhibition of the positive snow-albedo feedback by precipitation in interior Antarctica, Nature Climate Change, 2, 795, doi:10.1038/nclimate1590
• Zender, C.S. (2012), Snowfall brightens Antarctic future, Nature Climate Change, 2, 770.

Black Snow

When we burn fossil fuels or biomass, we produce CO2, one of the most important greenhouse gasses currently affecting global warming. During the process, however, not all fuel is burned completely, and some of the particles are released into the atmosphere as soot, of which the main component is black carbon. It's the same black stuff that clogs up chimneys or what's left over when you (accidentally) burn your food.

The closer you get to urban regions or factories, the more soot you find that has been emitted by cars or factories; but even in the remote polar regions, significant amounts of soot have been found in what we like to think of as pristine, white snow (Hansen & Nazarenko 2004). It has been transported there through the atmosphere.

The black carbon particles absorb heat when they float through the atmosphere, warming up their surroundings. Moreover, when they reach the surface through precipitation, they do the same thing there. When they fall on snow or ice, the warming process is amplified because the soot particles reduce the albedo. A lower fraction of the incoming solar radiation is reflected, heating up the ice, and inducing melt. A representation of this process can be found in the figure below.

The difference between incoming solar radiation on 'white' snow and snow with black carbon. Source: UK Met Office.

Researches have found that soot plays an important part in Arctic warming via this process. A study done by Mark Jacobson in 2010 found that the reduction in global albedo as a result of soot on snow and ice was between 3.3 and 5.2% (Jacobson 2010), with the strongest effects in the colder regions of the Northern Hemisphere such as Canada and northern Europe. It has been shown that the warming effects of black carbon in the Arctic might have been between 0.5 and 1 ºC (Ramanathan & Carmichael 2008) - which might not seem as a lot, until you realise that it could mean the difference between frosting or thawing.

The effects of soot on global warming are second only to CO2, and larger than many other greenhouse gasses such as methane, CFCs and nitrous oxide. It is estimated that the emission of black carbon is as much as 8 Teragrams annually (Ramanathan & Carmichael 2008), the equivalent of 1785 fully loaded Airbus A380s, the biggest passenger airplanes in the world. Most of the emissions come from North America and Europe, but developing industrial countries in Asia are quickly catching up.

This may sound all doom and gloom, but there is hope. Research has also shown that when soot is stopped being emitted into the atmosphere, its effects disappear within years (Ramanathan & Carmichael 2008; Jacobson 2010). Technologies already exist to accomplish that: for example, filters on diesel engines can prevent soot from being emitted. So compared to CO2, which stays in the atmosphere for hundreds of years, soot removal has a direct positive effect.

Additionally, soot is a health hazard, so taking it out of the atmosphere actively contributes to human well-being. This could be another incentive for policy makers to tackle black carbon emissions, so the Arctic can become pristinely white again.

Further reading
The Climate Change We Can Beat. By David Victor, Charles Kennel and Veerabhadran Ramanathan.

References

• Hansen, J., and L. Nazarenko (2004), Soot climate forcing via snow and ice albedos, PNAS, 101, 423, doi:10.1073/pnas.2237157100.
• Jacobson, M.Z. (2010), Short-term effects of controlling fossil-fuel soot, biofuel soot and gasses, and methane on climate, Arctic ice, and air pollution health, Journal of Geophysical Research, 115, D14209, doi:10.1029/2009JD013795.
• Ramanathan V. and G. Carmichael (2008), Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221, doi:10.1038/ngeo156

Arctic Defrost?

This September, the NSIDC (the US National Snow and Ice Data Center) revealed that the extent of the September sea ice in the Arctic this year was at the lowest point since measurements started in 1979 (a link to the press release can be found here). 

An image of the extent of sea ice in September 2012. The pink line is the average extent around this time over the period of 1979-2000 (NSIDC)

The record is in line with observations over the last decades, showing that there has been a steady decline in summer sea ice extent. Scientists believe that this trend is largely due to higher temperatures as a result of global warming.

A graph comparing sea ice extent between 2012, 2007 (the previous record-low) and the 1979-2000 average (from 30 October 2012 - NSIDC website)

Another observation is the decrease in thickness of the ice. The ocean water absorbs more heat, and slowly releases it when the sun sets - making it more difficult for the ice to regrow. Most of the ice is now only 1 or 2 years old, and is more susceptible to melting. Moreover, the disappearance of the ice means that less sunlight can be reflected back, resulting in warmer conditions. The positive albedo effect is working here as well.

It was previously thought from models from the IPCC report from 2007 that the Arctic might become ice free in summers by 2100. However, these recent data seem to indicate that the decline in ice is progressing faster than previously thought. Scientists now believe that summer ice might disappear completely from the Arctic region in a few decades.

I want to conclude this short update on the state of the Arctic sea ice with an informative clip from Climate Watch Magazine, where the sea ice has been animated based on the observations from 1979 onwards. It also shows the age of the ice in different colours - note how the 1 year old ice starts to dominate towards the end. It can be found here (I'd insert it - but it's a bit too big!).

In a following post, I'll give more attention to albedo feedback and ice, and I'll also discuss the effect of soot on ice reflectivity, something that's become more important in recent years.

To Plant or not to Plant? Reforestation and Albedo

For thousands of years, humans have cleared land for agriculture, cutting down trees to make space for croplands. Deforestation is an ongoing process; in the tropics especially, trees are cut down or even burned at rapid pace.

Forests play an important role in climate, especially in the hydrological and carbon cycles. Trees control humidity (and thus temperature) through evapotranspiration, and it has been found that deforested areas in the Amazon area have become both drier and warmer. Moreover, a growing forest fixes carbon from the atmosphere - cutting down trees releases it. Reforestation has been suggested as an option to sequester carbon, and thereby reduce the carbon dioxide in the atmosphere and counter global warming.

So how does albedo fit into this? As explained in earlier posts, different surfaces have a different albedo. A good example of this can be found in the figure below, derived from an article by Bonan in 2008. As can be seen, the albedo of forests is generally lower than that of other biomes. This is due to the fact that forests are quite dark and shady, and they do not reflect much of the solar radiation.

Surface albedo of different forest biomes (Bonan 2008)

The question scientists have been asking is whether this 'warming' effect offsets the cooling benefits of a forest. It seems as though this varies per type of forest. Basically, we can identify three types; tropical, temperate and boreal. It was found that evapotranspiration is very important in tropical forests. Warmer air can contain more water vapour and so more heat can be removed in that way (this is called latent heat transport). As said above, cutting down trees in Amazonia led to a warmer and drier climate. Additionally, rain forests grow rapidly and are able to store much carbon.

In boreal forests, snowfall also plays a role. Research has shown quite clearly that snow on bare ground has a higher albedo than snow on trees (which makes sense; imagine a forest covered in snow compared to a field. The trees will still show dark patches, whereas the field is much more uniformly covered). In addition to that, the evaporative effect of boreal vegetation is much smaller, because the air is generally colder. Boreal forests also grow slower, so carbon storage benefits from reforestation would require more time to take effect.

For temperate forests, the effects of albedo and evaporative forcing are unclear - and it is not certain whether replanting those forests might actually help in negating global warming.

In a model run by Giddard et al. in 2005, it was found that changes in surface (so either forestation or deforestation) particularly impact the higher latitudes in the Northern Hemisphere (also see the figure below). They found that the albedo "side-effect" of reforestation would lead to a net-warming on a century timescale, offsetting any benefits of carbon sequestration.

Modelled changes in albedo (Gibbard et al. 2005)

As such, it has been concluded that reforestation of tropical areas has some use in counteracting global warming, temperate forests little to no effect, and replanting boreal forests actually exacerbates it. Nevertheless, though it is tempting to draw such a one-on-one relation between forests, albedo and climate, trees and forests have other effects on climate as well, on various spatial and temporal scales, not to mention the importance of forests for biodiversity and human culture. There is much research needed in modelling all interactions between forests and the climate system before it can conclusively be said whether (and where) reforestation can help mitigate global warming.

Sources: 

• Bonan, G.B. (2008), Forests and climate change: Forcings, feedbacks and the climate, Science, 320, 1444, doi:10.1126/science.1155121.
• Gibbard, S., K. Caldeira, G. Bala, T. J. Phillips, and M. Wickett (2005), Climate effects of global land cover change, Geophys. Res. Lett., 32, L23705, doi:10.1029/2005GL024550.

Uneven Distribution: Local Albedo

At this point, it is good to note that the light from the sun doesn't hit the Earth's surface evenly.

Source: Nature Education 

Over some parts of the Earth, the same amount of sunlight covers a larger area due to the spherical shape of the planet (also see picture above). This is one of the principal drivers of climate differences between different latitudes, and why it's warmer at the equator than at the North Pole.

This relates to albedo as well: Even though snow may have an albedo of up to 0.8 for pristine snow, the amount of light reflected back is still not very much because it was never a lot to begin with. Conversely, a tropical forest is dark, shady and absorbs a lot of sunlight. Its albedo will never be more than 0.2. Nevertheless, because the amount of incoming sunlight at the equator is so high, the absolute amount of reflected light might be more than at the poles. Local differences such as these can impact the planet's total albedo.

On average, the albedo of the planet Earth is about 0.39, even though the oceans have a far lower albedo and cover most of the surface. Clouds are an important contributor: as white sheets floating in the sky, they play an important role in returning sunlight before it has even hit the ground (a situation that can only too often be observed in real life in London).

More will follow soon on clouds, forests, deserts and other local albedo factors.

Albedo Albedo Albedo: What's it all about?

This blog is about the albedo effect; according to the Oxford Dictionary, it is "the proportion of the incident light or radiation that is reflected by a surface, typically that of a planet or moon". That might sound more complex than it actually is. What it comes down to, is that some surfaces are more reflective than others.

Think of snow. Anyone who's walked around a pristine field of snow in the sun will remember barely being able to look at it. The snow reflects the sun. Other surfaces, for example asphalt, actually absorb the sunlight. They are not hard to look at on a bright day, but they can become very hot. There is a fairly simple property that governs the albedo of a certain surface, namely colour. Basically, lighter colours reflect more light than darker ones, which is why white snow is so much more difficult to look at than black asphalt.

For the planet, this means that there are some parts that act like a mirror, reflecting the incoming sunlight back into space. Ice sheets have a high albedo, meaning that they sent back much light, as do clouds. Dark ocean water, forests and most human built structures have a low albedo. They absorb more light than they reflect back.

Source: http://www.climatepedia.org/Albedo

The albedo is the percentage of light reflected. Of course, it makes sense that when we change the surfaces of the Earth, because of global warming or deforestation, the Earth's albedo will change as well. Less ice will mean less sunlight being reflected back into space, and more light being trapped on Earth - which means more heat as well and more global warming leading to more ice melting, leading to lesser albedo of the Earth's surface... In other words, through the albedo effect, global warming could potentially be exacerbated.