Tweet of the week - Arctic Report Card

In the last post we saw when the Antarctic and Arctic cooled enough to allow the formation of ice sheets. Now the Arctic is warming at an alarming rate. NOAA has an Arctic Report Card were it presents the state of the Arctic based on Snow covered days, albedo, ocean temperatures and even polar bears!

#Arctic warming twice the global average. Will #Santa have to move? http://t.co/Z6RzF6JWWh pic.twitter.com/1AZePNl8s9
— NOAA Climate.gov (@NOAAClimate) December 24, 2014

Holiday glaciations

In honor of the holidays, let's talk about the North (and South) Pole!

Santa realises that CO₂ plays an important role in the climate
 of his home. Source: Skeptical Science

We have seen that the Earth has suffered many changes along its history, and Antarctica and the Arctic are no exception. Starting from the fact that Antarctica was not always in the same place - it has been in the polar latitudes of the southern hemisphere only since the Cretaceous. And even when it reached this location, it was warm (DeConto & Pollard, 2003). It was around 34 million years ago at the Eocene-Oligocene boundary, that the ice sheets started covering the continent to about 50% of present day ice (Zachos et al., 2001). The likely cause was a decline in CO₂ levels in combination with favourable periods of the orbital cycles and ice albedo feedback, as DeConto & Pollard  (2003) found in their modelling results. 

The ice covered Arctic is much more recent, with the glaciation occurring 10 million years ago at the earliest, though it was not until 2.55 million years ago that there were "significant ice sheets". In this case the cooling was driven in part by lower CO₂ levels caused by tectonic changes. The uplift of the Himalayas would have increased the chemical weathering and in consequence drawing down more CO₂ than before. However, the orbital forcing played an important role for the intensification of the glaciation. Between 3.2 - 2.4 million years ago, the orbital forcing promoted cooler summers in the Northern Hemisphere, which allowed the ice sheet to grow rapidly (Maslin et al., 1998).

Tweet of the week - Losing ice

This week NASA tweeted this:

An increase of sun’s energy absorbed in the Arctic aligns with the decrease in sea ice: http://t.co/04sE2z74vp #AGU14 pic.twitter.com/h6eiov9X4P
— NASA (@NASA) December 17, 2014

The picture shows the change in sea ice (left) and in absorbed solar radiation (right) since the year 2000. As expected, areas with greater absorbed radiation show the greatest decrease in sea ice. This also means the albedo will be lower in the areas with less ice, causing even more heating in those areas. We've seen the ice albedo feedback before, in the Snowball Earth and the Ordovician mass extinction, but working the other way around: cooling promotes the ice to expand, so the albedo rises, reflecting more sunlight, which leads to more cooling. Now we are seeing the opposite: warming in the Arctic means there is more sea ice melting, lowering the albedo (exposing the dark sea), which absorbs more heat and there is more warming. 

Overland and Wang (2013) looked into different studies and methods regarding sea ice loss and found that the Arctic could be nearly ice free (less than 1 million km²) in summer by the first half of this century. One of these studies, by "trendsetters", found that if the current sea ice loss trend continues, this could happen as soon as 2020. Models predict a later date, around 2040 at the earliest. 

The end of the dinosaurs

Last week we saw the largest of the mass extinctions. This week we'll cover the most famous one: the Cretaceous-Paleogene extinction event! This case is different from the other ones we've seen*: the trigger came from outer space  The killer asteroid left behind the Chixculub crater in the peninsula of Yucatan, that has 100 km in diameter  (Schulte et al., 2010). It was identified and dated in 1992, confirming its age of ~65 million years, coinciding with the extinction (Kring, 2007).

Some drama before you read on...

This extinction event killed not only the dinosaurs, but 70-76% of all species (Jablonski, 1994)! But how exactly did the asteroid accomplish it?

Well, once the asteroid hit the Earth, the initial consequences were incredibly strong earthquakes (models suggest magnitudes over 11!) and tsunamis. (Schulte et al., 2010). However, since this blog is oriented towards climate changes and environmental effects, let's center on the massive amounts of material ejected around the planet.

First, the impact site had anhydrite (CaSO₄), so the ejected material  contained sulfates, that once in the stratosphere would reflect the sunlight, causing cooling of the Earth's surface and limiting photosynthesis. The sulfates, dust and soot that reached the stratosphere may have remained there for a year (Kring, 2007). Schulte et al., (2010) however says that the cooling caused by the sulfate aerosols may have lasted for decades, lowering the temperature by 10°C.

Another effect was acid rain, caused by "shock-heating" of the atmosphere during the impact and mainly by the raining down of the ejecta. This heating produced NOx, which in addition to the sulfates in the debris, contributed to acid rain falling up to a few years after the impact (Kring, 2007).

The impact also caused wildfires. Though the extent is still not known, there is evidence from the soot recovered that ~10⁴ GT of CO₂ and ~10² GT CH₄ were released from these wildfires. The impact itself added CO₂, CH₄ and H₂O to the atmosphere. These greenhouse gases can remain more time in the atmosphere than the sulfates and dust, so a warmer period may have followed after the initial cooling.  (Kring, 2007).

The impact as cause of the extinction is the most widely accepted, but it is important to note that at the time of this event there was something else going on. Just like at the end Permian, there was massive volcanic activity that went on for about 1 million years. The Deccan flood basalt eruptions were located in present day India. However, as Schulte et al., (2010) mentions, the impact event and the volcanic event were magnitudes apart - the former injected up to 500 Gt of sulfur to the atmosphere almost instantaneously, while the latter contributed with up to 0.5 Gt of sulfur per year.

*Some studies suggest that this also caused other extinctions, like the end Permian event, with Becker (2004) presenting the Bedout crater as possible evidence, but it is still disputed.

Tweet of the Week - What if?

The tweet of the (last) week is thought provoking. What would happen if all life disappeared from our planet? Not just us humans, but all animals and plants...

Life disappears from the Earth. What happens then? http://t.co/hhN3dpVQBG pic.twitter.com/mGYQqXlvpg

— New Scientist (@newscientist) December 12, 2014

As we have seen, life appearing had a great impact on the planet: oxygen levels rose in the atmosphere and reduced CO₂, causing the Earth's temperature to plunge.  If we all disappeared, the opposite would happen: O₂ levels would fall and CO₂ would be on the rise again, causing great warming. 

Other possible changes? Without plants, precipitation patterns would be altered, causing the continents to be drier and hotter. The ozone layer would also be in trouble with the falling levels of  O₂. 

The article is very interesting, going through some other changes that could happen. It really shows how life is deeply connected with the climate!

End Permian Mass Extinction

Finally, the big one. Around 90% of all species went extinct: 70% of land vertebrates and 80-96% marine animals (Chen & Benton, 2012). After millions of years of living beings surviving the challenges of the changing Earth, what happened that almost wiped them out?

Fig. 1: A marine ecosystem before and after the mass extinction. Chen & Benton, 2012. © John Sibbick

The end Permian extinction event was 252 million years ago and it happened "quickly", in less than 100,000 years (Shen & Bowring, 2014). The exact causes are still debated, though there is one that stands out. This time the culprit seems to be volcanism (Payne & Clapham, 2012) - though not a lone volcano erupting, but massive eruptions, flooding extensive areas with over 5 million km³ of basalt. The eruption of the Siberian traps, located in central Russia, went on over a period of 1 to 2 million years (Shen & Bowring, 2014). The volcanic activity released huge amounts of CO₂ to the atmosphere; this could have even been increased due to interactions with organic deposits. Svensen et al. (as cited by Payne & Clapham, 2012) calculated that over 30,000 GT of carbon was released to the atmosphere! As you probably expect, there is evidence for a rise in global temperatures of around 8°C (Shen & Bowring, 2014). 

However, not only did the CO₂ influence the temperature; it likely caused ocean acidification and carbonate saturation. This would affect the marine animals, specially those with shells, leaving them without refugia (Payne & Clapham, 2012). 

Another mechanism involved in the extinction was marine hypoxia/anoxia, perhaps caused by the excessive chemical weathering of the Siberian traps, releasing phosphorous to the oceans (Payne & Clapham, 2012). The excess nutrients would have promoted eutrophication and hypoxia.

The emissions from the eruptions would have included not only CO_2, but sulfur, HCl and CH₃Cl, among others. Black et al. (2014) used a model to assess the effect of these emissions on the ozone layer and acid rain with different eruption scenarios. They found that the ozone layer could have been depleted up to 30% due to HCL and up to 67% due to CH₃Cl when these emissions reached the atmosphere. The effects would have been felt worldwide - increased UV B radiation with mutagenic effects. For the acid rain they considered a CO₂ concentration 10 times higher than present. This effect alone would have meant acid rain with a pH of around 4, and it would have been present worldwide. Combined with sulfate, for a 1 year eruption of 2400 km³, the pH may have dropped as low as 2, with the effects mainly in the Northern hemisphere (the location of the traps).

It took about 8-9 million years for life to make a full recovery (Chen & Benton, 2012), after almost being wiped out in 100,00 years. But this mass extinction paved the way for a new revolution of life, and the soon to begin age of the reptiles.

RIP
520 - 252 million years ago
Source: Wikipedia.com

Tweet of the week - The weather in 2050

This week, as part of the UN Climate Change Conference in Lima, the WMO has been releasing a daily video with a weather report of a day in 2050 for different countries. These reports are based on a climate change scenario where we keep increasing our greenhouse gas emissions - and they are quite alarming!

#Weather2050 reports. Today from Vietnam. Climate change is already happening #COP20 http://t.co/qeGt8Qrnby pic.twitter.com/j1eeWGzDYO
— WMO | OMM (@WMOnews) December 3, 2014

You can find all the videos (including an earlier batch of videos released in September during the UN Climate Summit) in the WMO website.

Here is the one for Spain, remembering the 2010 heatwave with some "nostalgia".

Faint Young Sun Paradox

In a couple of posts I have mentioned that the sun was fainter in the past. As you might imagine, it had an influence on the climate of the early Earth.

Sagan and Mullen presented this dilemma in 1972. 4.6 billion years ago the Sun was 30% fainter than today. This would mean, considering atmospheric composition and albedo to be the same as in present day, the temperature on the Earth would have been -10°C (263 K). Temperatures would have reached -2°C, the freezing point of seawater, around 2.3 billion years ago (Fig. 1).

Fig. 1: Without greenhouse gases the temperature would have been even lower (see the "Effective temperature"). Their influence is so important, that even today, with a stronger Sun, our planet would still be freezing. The "CO2-H2O greenhouse" represents our present conditions. Chyba, 2010. Simplified from Sagan and Mullen, 1972

As we have seen in past posts, by 2.3 billion years, there was life on the planet (it had already started with the small task of helping oxygenate the planet). Sagan and Mullen (1972) saw the inconsistency between the theory and the evidence available - it is unlikely that life could appear without the presence of liquid water and, even back in 1972, there was evidence for liquid water 3.2 billion years ago.

And the solution is...

Nothing conclusive so far, though not through lack of trying. There have been several different atmospheric compositions proposed to counter the effects of the young Sun. Sagan and Mullen (1972) themselves proposed an atmosphere with ammonia (NH3) as the greenhouse gas responsible of warming the planet (Fig. 1, "10^-5 NH3 greenhouse" line). However, this has since been disproved. As Chyba (2010) mentions, the NH3 would have been photolyzed by the UV radiation from the Sun converting it into N2, thus eliminating its greenhouse effect. Haqq-Misra et al. (2009) proposed an atmosphere with CH4, C2H6, CO2 and H2O. Rosing et al. (2010) argued that the high greenhouse gas concentrations required by this model (as by others) was not consistent with the geological data, and proposed that the deciding factor was not the atmosphere, but the albedo. The albedo would have been much lower due to the presence of huge oceans, causing a higher absorption of heat which together with the presence of greenhouse gases would have been enough to keep the planet warm. 

In the end everything seems to go back to greenhouse gases such as  CH4 and CO2  as the reason the Earth was warmer, though the exact composition of the atmosphere remains unknown. 

The Ordovician Mass Extinction

Finally it is time to leave behind the Precambrian and start with our current eon, the Phanerozoic! Bear in mind that it started 541 million years ago so we still have a long way to go before we make it to today. Besides, we haven't even talked about dinosaurs (or at least when they died out)!  

Here, have a dino pic anyway. Source: Smithsonian.com

Speaking of dying out... we are now entering a time of mass extinctions. So far there have been 5 mass extinctions (the Big Five!) and as you may have seen in the news, there is talk of whether we are going through the sixth mass extinction or not.  

Wait a minute, this blog deals with climate, not extinctions! True, but life and climate are closely connected. So if there is a great extinction, it is likely there was a change in the environment at the time, though the exact causes may vary.

Trilobite. Source: Wikipedia.com

The first of the Big Five mass extinctions happened at the end of the Ordovician (hence its name, "Late Ordovician Mass Extinction"), about 443 million years ago. By this time life had evolved to more complex forms and taken over the oceans. There were echinoderms, sponges, the all famous trilobites, among many others (Harper, 2006). It was a generally long warm period with sea levels that were the highest of the Paleozoic (Munnecke et al., 2010). The continents were distributed from the south pole to low latitudes and most of the northern hemisphere was covered by an ocean. Gondwana was the largest of these continents.

What happened?

There was another glaciation. This time, due to the configuration of the continents, the glaciation was limited to the southern hemisphere (Sheehan, 2001) and not as extreme as the Snowball Earth events, but enough to lower the sea level and reduce the area of available habitats for marine life. In addition it directly affected the taxa that were not adapted to cold temperatures. As a result, 61% of the marine genera went extinct (Finnegan et al., 2012).

Box 1. Source: Munnecke et al., 2010 

Fig. 1 shows reconstructions of atmospheric CO2 and O2 during the Paleozoic, with the end of the Ordovician marked in red. There is no certainty of these, with several reconstructions being shown in the graphs, but it seems to be clear that the O2 were not yet as high as present day (<21%) and CO2 was possibly up to 10 times present day values (for the latest value, check out the side bar!), around 4000 ppm (Munnecke et al., 2010).

Fig. 1: Atmospheric CO2 and O2 during the Paleozoic.
Munnecke et al., 2010  

The mechanism proposed to explain the decrease in CO2 levels during the late Ordovician was an increased silicate weathering caused by a mountain building period (Sheehan, 2001; Munnecke et al., 2010). This weathering consumed atmospheric CO2 helping reduce it until it reached a critical threshold that allowed the growth of ice sheets and lowering of the sea level. This threshold is debated, but generally accepted as around 8 times present day levels, with Munnecke et al. (2010) giving a value of around 3000 ppm. It is not comparable to current conditions (see Box 1) and is MUCH MUCH higher than what would be required today to melt the ice caps.

Once again the CO2 levels decreased, allowing the temperatures to drop and ice to grow. Then back to the usual suspect: as the ice expanded it provided a positive ice albedo feedback, reinforcing the cooling and further growth of ice. Lower sea levels also exposed more land subject to weathering that contributed to continue reducing atmospheric CO2. However once the ice covered the land, the weathering that had been the mechanism to drawdown CO2 was reduced, leading to a gradual build up of CO2 in the atmosphere once again.