Showing posts with label reflectivity. Show all posts
Showing posts with label reflectivity. Show all posts

Friday, June 5, 2015

High Temperatures in the Arctic


The images below illustrate extremely high temperatures forecast to hit Russia on June 6, 2015, as also discussed in the previous post.


A temperature of 29.4°C (84.92°F) is forecast for the location at the green circle for June 6, 2015. The location is close to the Arctic Ocean and to rivers ending in the Arctic Ocean, as also shown on the image below.


The location, at a latitude of 66.48°N, is approximately on the Arctic Circle, which runs 66°33′45.8″ north of the Equator. North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once a year.


The many hours of sunshine make that, during the months June and July, insolation in the Arctic is higher than anywhere else on Earth, as shown on above image, by Pidwirny (2006).
Insolation, with contour labels (green) in units of W m−2

The size of the June snow and ice cover is so vitally important as insolation in the Arctic is at its highest at the June Solstice.

The Wikipedia image on the right calculates the theoretical daily-average insolation at the top of the atmosphere, where θ is the polar angle of the Earth's orbit, and θ = 0 at the vernal equinox, and θ = 90° at the summer solstice; φ is the latitude of the Earth.

The calculation assumed conditions appropriate for 2000 A.D.: a solar constant of S0 = 1367 W m−2, obliquity of ε = 23.4398°, longitude of perihelion of ϖ = 282.895°, eccentricity e = 0.016704.

Snow and ice cover on land can take up a large area, even larger than sea ice. In May 2015, the area of snow extent on the Northern Hemisphere was 17 million square km, while sea ice extent in May 2015 was below 13.5 million square km. 

Northern Hemisphere snow, May 2015. Credit: Rutgers University Global Snow Lab
The chart below shows the decline of snow cover on land on the Northern Hemisphere in Spring over the years. 

Credit: Rutgers University Global Snow Lab
High temperatures over the Arctic Ocean are heating up the snow cover on land and the sea ice from above. High temperatures also set the scene for wildfires that can emit huge amounts of pollutants, including dust and black carbon that, when settling on the sea ice, can cause its reflectivity to fall. Rivers furthermore feed warm water into the Arctic Ocean, further heating up the sea ice from below. 

The image below shows Arctic sea ice extent at June 3, 2015, when Arctic sea ice extent was merely 11.624 million square kilometers, a record low for the time of the year since satellite started measurements in 1979. 



Sea ice melting occurs due to heat from above, i.e. absorbed sunlight. Once the sea ice is gone, energy from sunlight that previously went into melting and transforming ice into water, will instead go into warming up the Arctic Ocean and the sediments under the seafloor.

In addition, sea ice is also melting due to heat from below. Much of this heat is carried by the Gulf Stream and by rivers into the Arctic Ocean. Once the sea ice is gone, all this heat will go into warming up the Arctic Ocean and the sediments under the seafloor.

The sea ice acts as a heat buffer by absorbing energy in the process of melting. In other words, as long as there is sea ice, it will absorb heat and this will prevent this heat from raising the temperature of the water in the Arctic. Once the sea ice is gone, this latent heat must go elsewhere.

As the sea ice heats up, 2.06 J/g of heat goes into every degree Celsius that the temperature of the ice rises. While the ice is melting, all energy (at 334J/g) goes into changing ice into water and the temperature remains at 0°C (273.15K, 32°F).

Once all ice has turned into water, all subsequent heat goes into heating up the water, at 4.18 J/g for every degree Celsius that the temperature of water rises.

The amount of energy absorbed by melting ice is as much as it takes to heat an equivalent mass of water from zero to 80°C. The energy required to melt a volume of ice can raise the temperature of the same volume of rock by 150º C.
Decline of Arctic sea ice means that a lot more heat will be absorbed by the Arctic Ocean.



Thick sea ice covered with snow can reflect as much as 90% of the incoming solar radiation. After the snow begins to melt, and because shallow melt ponds have an albedo (or reflectivity) of approximately 0.2 to 0.4, the surface albedo drops to about 0.75. As melt ponds grow and deepen, the surface albedo can drop to 0.15, while the ocean reflects only 6% of the incoming solar radiation and absorbs the rest.

As Professor Peter Wadhams, University of Cambridge, once calculated, a collapse of the sea ice would go hand in hand with dramatic loss of snow and ice cover on land in the Arctic. The albedo change resulting from the snowline retreat on land is similarly large as the retreat of sea ice, so the combined impact could be well over 2 W/sq m. To put this in context, albedo changes in the Arctic alone could more than double the net radiative forcing resulting from the emissions caused by all people of the world, estimated by the IPCC to be 1.6 W/sq m in 2007 and 2.29 W/sq m in 2013.

Professor Peter Wadhams on albedo changes in the Arctic

Update June 8, 2015: The website at earth.nullschool.net shows that over the past few days temperatures over 30°C (86°F) were reached at several locations over rivers ending up in the Arctic Ocean.

The animation below, by ClimateReanalyzer, shows the heat wave and the storm that hit the Arctic recently.


This animation shows the current GFS model 8-day forecast for the Arctic for six meteorological parameters (precip/cloudcover; wind, pressure, precipitable water, temperature, temperature anomaly). The forecast begins with an impressive storm twirling around the North Pole with 10-meter winds peaking around 55 km/h (~35 mi/h), which fades as the low pressure breaks down. The storm is coupled to an early season heat wave that hit Siberia this week with the development of a high amplitude ridge in the jet stream.In mid August 2012, a comparable storm churned up the sea ice and contributed to the record minimum ice extent that emerged in September. Arctic sea ice is more resilient to wind in early June when it is still relatively thick and compacted than it is in mid August towards the end of the melt season. This current storm is therefore unlikely to have the same impact as the Aug 2012 storm. But the event is worth mentioning nonetheless.

Posted by Climate Reanalyzer on Sunday, June 7, 2015




Thursday, January 10, 2013

Dark Snow Project - Research into soot on Greenland

Fossil fuel combustion creates carbon emissions that increase atmospheric thickness, warming climate. The occurrence of wildfire increases with climate warming, increasing soot loading of the atmosphere. Some of this soot is transported through the atmosphere and is deposited on glaciers, lowering their reflectivity, increasing solar energy absorption, increasing melt rates.
image from DarkSnowProject.org

In parts of Greenland where winter snow loss during each melt season exposes impurity-rich bare ice, the surface reflectivity drops from 85% to 30%. Consequently, most of the 24-hour sunlight goes into ice melt. In this Dark Zone, the impact of soot manifests strongest in a self-reinforcing feedback loop that research by Jason Box has shown to have doubled melt rates in the past decade.

High on the inland ice sheet where melting is rare, satellite data show surface darkening making the researchers suspect that wildfire and industrial soot are to blame. Darkening here promotes snowpack heating, bringing earlier melt, keeping melt going longer. Here is where this feedback is changing the ice sheet in surprising ways, leading to complete surface melting in year 2012.



To measure the extent to which soot particles enhance melting, Jason Box is organizing a Greenland ice sheet expedition for 2013. The Dark Snow Project expedition is to be the first of its kind, made possible by crowd-source funding.



References

Fire and Ice: Wildfires Darkening Greenland Snowpack, Increasing Melting (News Release from Byrd Polar Center)
http://bprc.osu.edu/~jbox/DS/20121205_news_release_CALIPSO_etc.pdf

- The DarkSnowProject
http://darksnowproject.org

-Video: Sampling Greenland, the Dark Snow Project, by Peter Sinclair, produced at Greenman Studio, Midland, MI.
http://www.youtube.com/watch?v=vT6H7HPWkqU

- Where there’s fire there’s smoke - Blog by Jason Box, the Meltfactor.org


Further reading

- Greenland is melting at incredible rate
http://arctic-news.blogspot.com/2012/07/greenland-is-melting-at-incredible-rate.html

Friday, December 28, 2012

Albedo changes in the Arctic

How global warming and feedbacks are causing huge albedo changes in the Arctic.

Snow cover decline

Decline of the snow cover on land in the northern hemisphere is accelerating, as illustrated by the image below and the image underneath on the right. (1)


Image credit: Rutgers University
Fresh snow can have an albedo as high as 0.85, meaning that up to 85% of the sunlight falling on snow can get reflected back into space. As the snow melts, its structure changes making it less reflective, i.e. its albedo will go down, to as low as 40%. (2)

As a result, more sunlight gets absorbed, accelerating the melting process. Eventually, where snow melts away, spots of bare soil become exposed, and dark wet soil has a very low albedo, reflecting only between 5% and 15% of the sunlight. Thus, even more sunlight gets absorbed and the soil's temperature increases, causing more of the remaining snow to melt. (2)

Changes in vegetation can further accelerate this process. Russia's boreal forest - the largest continuous expanse of forest in the world - has seen a transformation in recent years from larch to conifer trees. Larch trees drop their needles in the fall, allowing the vast, snow-covered ground in winter to reflect sunlight and heat back into space and helping to keep temperatures in the region very cold. But conifers such as spruce and fir retain their needles, which absorb sunlight and increase the forest's ground-level heat retention. (3)

Albedo, from Wikipedia
A conversion from larch to evergreen stands in low-diversity regions of southern Siberia would generate a local positive radiative forcing of 5.1±2.6 W m−2. This radiative heating would reinforce the warming projected to occur in the area under climate change. (4)

Tundra in the Arctic used to be covered by a white blanket of snow most of the year. However, as the landscape is warming up, more trees and shrubs appear. Scientists who studied part of the Eurasian Arctic, found that willow and alder shrubs, once stunted by harsh weather, have been growing upward to the height of trees in recent decades. They now rise above the snowfall, presenting a dark, light-absorbing surface. This increased absorption of the Sun's radiation, combined with microclimates created by forested areas, adds to global warming, making an already-warming climate warm even more rapidly. (5 & 6)

Furthermore, encroachment of trees onto Arctic tundra caused by the warming may cause large release of carbon to the atmosphere, concludes a recent study. This is because tundra soil contains a lot of stored organic matter, due to slow decomposition, but the trees stimulate the decomposition of this material. (7)


Sea ice decline

In the Arctic, sea ice volume has fallen dramatically over the years, as illustrated by the image on the right. The trend points at 2014 as the year when Arctic sea ice will first reach zero volume for some time during that year. (8)

The Arctic Ocean looks set to be ice-free for a period of at least three months in 2015 (August, September and October), and for a period of at least 6 months from the year 2020 (June through to November). (9)

Decline of the Arctic sea ice is accelerating, due to numerous feedbacks. As the Arctic atmosphere warms up, any snow cover on top of the ice will melt away ever quickly, decreasing the surface albedo and thus reinforcing the warm-up. As melt ponds appear on top of the ice, the albedo will drop even further.

Sam Carana's Diagram of Doom pictures ten feedbacks that jointly work to accelerate sea ice decline. (10)

The image below shows the three areas where albedo change will be felt most in the Arctic, i.e. sea ice loss, decline of albedo in Greenland and more early and extensive retreat of snow and ice cover in other areas in the Arctic. (8)

Big changes in the Arctic within years, by Sam Carana


References

1. Northern Hemisphere Snow Cover Anomalies 1967-2012 June, Rutgers University
climate.rutgers.edu/snowcover/chart_anom.php?ui_set=1&ui_region=nhland&ui_month=6

2. Albedo, Albedo Change blog
albedochange.blogspot.com/2009/02/albedo-change.html

3. Shift in Northern Forests Could Increase Global Warming, Scientific American, March 28, 2011
scientificamerican.com/article.cfm?id=shift-northern-forests-increase-global-warming

4. Sensitivity of Siberian larch forests to climate change, Shuman et al., April 5, 2011, Wiley.com
onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2011.02417.x/abstract

5. Warming turns tundra to forest
ox.ac.uk/media/news_stories/2012/120604.html

6. Eurasian Arctic greening reveals teleconnections and the potential for structurally novel ecosystems, Macias-Fauria et al., 2012
nature.com/nclimate/journal/v2/n8/full/nclimate1558.html

7. Expansion of forests in the European Arctic could result in the release of carbon dioxide, University of Exeter news, June 18, 2012
exeter.ac.uk/news/featurednews/title_214902_en.html

8. Big changes in the Arctic within years, Sam Carana, October 26, 2012, Arctic-News blog
arctic-news.blogspot.com/2012/10/big-changes-in-arctic-within-years.html

9. Getting the Picture, Sam Carana, August 2012, Arctic-News blog
arctic-news.blogspot.com/2012/08/getting-the-picture.html

10. Diagram of Doom, Sam Carana, August 2012, Arctic-News blog
arctic-news.blogspot.com/2012/08/diagram-of-doom.html


Further reading

- Albedo change in the Arctic
arctic-news.blogspot.com/2012/07/albedo-change-in-arctic.html

- Greenland is melting at incredible rate
arctic-news.blogspot.com/2012/07/greenland-is-melting-at-incredible-rate.html

- Albedo change in the Arctic threatens to cause runaway global warming
arctic-news.blogspot.com/2012/10/albedo-change-in-the-arctic-threatens-to-cause-runaway-global-warming.html