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.

Let's talk climate change

In between talking about billions and millions of years ago, I woud like to remind you that on Monday, December 1st the UN Climate Change Conference will begin in Lima, Peru!  

Let the games begin... #COP20 in Lima :-) pic.twitter.com/W8GiSRYRpj

— #COP20 Lima (@LimaCop20) November 28, 2014

As we have seen, our climate has changed before for different reasons, ranging from tiny microorganisms to huge continents, but generally changes have taken a very long time - thousands or millions of years. Today we are seeing an accelerated climate change, in a very short timescale.

In its latest report the IPCC said:

"Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems."

So, it is also up to us to make the necessary changes to try to mitigate the effects as much as possible. This meeting will be important leading up to Paris 2015, and will mark the trend of where we are heading to in the future.

If you want to follow the action, http://newsroom.unfccc.int/lima/ is a good place to start. Oh, and of course there is an app!
Google play
Appstore

A frozen planet - II

Planet vitals:

- Average temperature: -20 to -50°C

- Pack ice thickness: 500-1500 m
(Kirschvink, et al., 2000)

Source: BBC

How did the Earth fall into such extreme conditions?

First of all, the young Sun was weaker than in the present day (Tziperman et al., 2011; Young, 2012). Perhaps the only reason the Earth was not under permanent icehouse conditions in the past was the high concentration of greenhouse gases (mainly CO2 and CH4) in the atmosphere. This means that with a reduction in greenhouse gases, the Earth would cool down (Young, 2012 and Tang and Chen, 2013).

CH4 levels fell with the rise of oxygen during the Great Oxidation Event, eliminating one of the greenhouse gases. CO2 reduction was also related with this event, since microorganisms would consume it during photosynthesis (Tang and Chen, 2013). However, weathering is also an important mechanism to drawdown CO2. This is where the supercontinents come in. Young (2012) notes that the Snowball events at the beginning and end of the Proterozoic coincided with the presence of supercontinents. These are very thick and rise higher with respect to sea level than smaller continents, exposing more surface subject to subaerial weathering, so more CO2 can be "buried", effectively reducing atmospheric CO2. Fig. A is a simplified diagram that shows the conditions that made the Snowball Earth events (marked with red arrows) possible. For the first event there was low solar luminosity and a drop in CO2, caused by increased weathering due to the Kenorland supercontinent. Once it broke up, the CO2 drawdown was reduced, but maintained a downward trend (the atmospheric O2 was still rising). Solar luminosity also progressively increased, preventing icehouse conditions despite the lowering CO2. However, once Rodinia was formed, the CO2 weathering increased once again, breaking the balance and plunging the Earth into another glaciation.

Fig. A: Relation between glaciations and supercontinental cycle. Young, 2012

Once the ice began to expand across the planet, there was no stopping it; a runaway ice-albedo effect would be triggered when sea ice crossed between 30° to 40° latitude, causing the whole planet to be covered in ice (Tziperman et al., 2011).

How did it end?

It seems like there is no escape. With the runaway ice-albedo effect, the cooling effect is reinforced endlessly. However, as shown in fig. B, weathering is prevented by the ice cover, allowing CO2 levels to build up again, warming the planet and allowing the ice to melt (Young, 2012). Volcanoes would be the source of CO2 (Hoffman et al., 2002) and Kirschvink et al. (2000) estimated it would take roughly 35 million years to end a Snowball event.

Fig. B: Feedback loop causing alternation between icehouse and greenhouse conditions. Young, 2012.

A frozen planet

After the Archaean-Proterozoic transition, where we left off, the Earth went through severe glaciations that would put the last ice ages to shame. These glaciations lasted from 100 to 200 million years at the beginning and the end of the Proterozoic (Hoffman, 2002). The extent of these glaciations is still debated, with some arguing for Snowball Earth events (Hoffman, 2002), while others suggest the evidence is inconclusive for such widespread events (Young, 2012).

The Huronian glaciations occurred at the beginning of the Proterozoic (2.45-2.22 billion years ago), with at least 3 being identified (Young, 2012). The Sturtian (715 million years ago) and Marinoan (635 million years ago) glaciations occurred at the end of the Proterozoic. 

Fig. A: Current location of Neoproterozoic glacial deposits, and
their estimated original latitudes. Hoffman, 2002

The evidence for these events is found spread around the modern day continents, mainly in the form of glacial deposits, like ice rafted debris. By estimating their original latitudes (figures A and B), it was found that many of these glacial features were formed near the equator (Hoffman, 2002), leading to the idea of frozen world, or Snowball Earth.

Fig. B: Location of Sturtian glacial deposits on the
supercontinent Rodinia. Young, 2012

However, as mentioned before, the extent of these glaciations is not yet clear. A ‘Slushball Earth’ scenario, with open water, is an alternative. Among the arguments against a Snowball Earth (Young, 2012) is that without open water, the hydrological cycle would basically “shut down” and additionally there is evidence for glacial advance and retreat cycles. Another important consideration is how or where microorganisms could have survived in a completely frozen world - photosynthetic organisms had already evolved as discussed previously. Hoffman (2002) argues a weak hydrological cycle would be present and enough for snow to accumulate slowly, eventually allowing for glaciers to flow. Regarding the microorganisms, he says that there were many places where they could survive, like transient meltwater ponds or hot springs around volcanic islands. He argues that microorganisms did not only survive, but benefited from these harsh conditions, since it was only after the Neoproterozoic glaciations that there was an increase in their diversity, and the appearance of the first macrofossils came soon after.

Fig. C: Macrofossil from the Edicaran period (635-542 million years ago). Source: Wikipedia 

In the next post we will explore the possible causes that led to these extensive glaciations and why they occurred so long ago!  

Vulnerability

Along with the regular posts, I will be posting shorter* ones about climate change today. Let's start with vulnerability!

A few days ago the UNEP tweeted an interesting map of a ranking of countries by vulnerability.

This map compares 3 lists of countries ranked by #climatechange vulnerability. Surprised? http://t.co/TmY5EusCti pic.twitter.com/B4Dpj6chLl
— UN Environment (@UNEP) November 1, 2014

Well yes, I was surprised. 

First, because there is a lot of discrepancy between the indices, with several countries appearing among most vulnerable and least vulnerable lists simultaneously. The second thing I noticed is that Peru appears to have no data. 

So let's look at the three indices used to make the map. 

DARA Climate vulnerability monitor

Uses 34 indicators to link the effects of climate change with the economic, ecological and social costs. In the social aspect it includes deaths and number of people affected.

Notre Dame Global Adaptation (GAIN) Index

Their vulnerability score is based on 36 indicators. From the page: "ND-GAIN measures overall vulnerability by considering six life-supporting sectors: food, water, health, ecosystem service, human habitat, and infrastructure."  The total ND GAIN score includes each countries readiness along with the vulnerability.

Germanwatch's Global Climate Risk Index

Uses information of the social and economic effects of extreme weather events that occurred between 1990 and 2009. It does not consider indirect effects or events, such as glacier melting. In the social aspect, it only includes deaths associated with extreme weather events, not number of people affected. It produced two rankings, one for the most affected countries from 1990-2009 and another only for 2009. 

Basically, even though the three indices are related to climate change vulnerability, they are not looking at the same things and are not necessarily comparable. Also, each methodology will have certain limitation based on the data used, so that can explain why the results are not as expected. If something doesn't make sense at first, look at the methodology and data used! This doesn't mean an index is wrong or right, it is just that they have a different goal and will use different data. Each webpage linked has a lot of very interesting information, so check them out.

I must note that I still don't understand why Peru appears as "data non available", since it was analyzed in all three indices (ND GAIN: 71 out of 180 in the vulnerability index; Germanwatch: 47th most affected from 1990-2009 and 51st in 2009; DARA CVM: severely vulnerable). What this shows is that we must all be prepared! Every country will probably be affected in one way or another and it is in our hands NOW to take the necessary measures to be ready.

*Did I say shorter at the beginning?

A day on Earth… 2.4 billion years ago

When the Earth formed, its atmosphere was very different from today. In fact, in the beginning it almost had no atmosphere at all. It took millions of years for the planet to cool down enough to start building up an atmosphere composed mainly by water vapour, carbon dioxide and nitrogen. Notice anything missing?

Exactly, no oxygen! The components of this early atmosphere came from the gases released from the molten rock. And one important detail: there was no life yet.

An early Earth may have looked like this. Peter Sawyer / Smithsonian Institution

The oxygen that is found in the atmosphere (O2) comes primarily from photosynthesis. The earliest evidence for life on Earth dates to about 1 billion years after the Earth formed. Early organisms were anaerobic, likely using methanogenesis or anoxygenic photosynthesis. Neither of these mechanisms release oxygen, but rather methane or sulphur components and water (Catling & Claire, 2005). The exact moment that oxygenic photosynthesis appeared is debated, especially considering that many fossils found are poorly preserved (Sessions et al., 2009). However, to get an idea, in fig. A there is a timescale showing the ages of fossils found, and possible moments when photosynthesis could have appeared, since cyanobacteria produce O2.

Fig. A: Timescale and ages of fossils. Sessions et al. (2009)

So what happened 2.4 billion years ago?

The oxygen levels in the atmosphere increased in what is known as the "Great Oxidation Event" (fig. B). The atmospheric oxygen went from 0.001% of present day levels to 5-18% by the time everything stabilised, some 1.8 billion years ago (Sessions et al., 2009). There was a second event that finally took the oxygen levels to present day levels about 500 million years ago.

Fig. B: A simplified view of the evolution of oxygen in the atmosphere. Sessions et al. (2009)

Now, cyanobacteria had already been doing photosynthesis and producing oxygen for some time (at least some 300 million years), but it wasn't building up in the atmosphere, at least not in the amounts shown by the geological records. This means that around this time there must have been a change: either there was an increase in the production of oxygen, or a change in the oxygen sinks to allow the O2 to remain in the atmosphere.  

One hypothesis relates the Great Oxidation Event with the changes in the tectonics of the planet that happened around the same time. Continental land masses stabilised towards the end of the Archaean (~2.5 billion years ago). Kump (2007) suggests that an increase in subaerial vulcanism is related with a reduced sink for oxygen, since before the emergence of continents most volcanic activity was submarine and more reducing (used up more oxygen). This is a compelling hypothesis because changes on the Earth of this scale (mayor continental change and substantial increase in atmospheric oxygen) happening at the same time are generally more than a simple coincidence. 

Why is this important?

Besides the fact that WE need oxygen to breathe, it is likely that complex multicellular life was able to evolve thanks to this: aerobic respiration is much more efficient than anaerobic fermentation (Sessions et al., 2009).

Catling & Claire (2005) also mention a couple of important things to consider:

• An early ozone layer in the stratosphere emerged once there was enough oxygen in the atmosphere (approximately 0.1% present day levels).
• Methane levels where high (simulations indicate 100 to 1000 times higher than present day), which kept the Earth warm, despite a "weaker" Sun. The rising atmospheric oxygen caused a reduction in methane, cooling the Earth down significantly, perhaps even causing  Snowball Earth conditions.  

This is only one of many mayor changes the Earth has gone through. In this case the intervention of newly appeared life played an important role in changing the atmosphere. However, as we will see in the next posts, one change can be followed by others that are unexpected.

------

Other bibliography:

Tarbuck, E. and Lutgens, F. (1997). Earth science. Upper Saddle River, NJ: Prentice Hall.

First, some perspective

Before we start exploring the different events of Earth's history, it is important to try to understand just how old the Earth is. I say try, because for most of us it is a bit hard to understand a timescale of a million years, and even worse, a billion years!

The Earth is estimated to be between 4.51 to 4.55 billion years old (Dalrymple, 2001). This chart by the International Commission on Stratigraphy shows the units of time in which the geological time scale of our planet is divided and will be useful once we start dealing with past events.

 Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) 
The ICS International Chronostratigraphic Chart. Episodes 36: 199-204.

However, to get a better understanding of the massive timescale we are talking about, this "Cosmic Calendar" is quite useful. On it, the Big Bang happened on the first second of January 1st. Considering the universe is around 13.8 billion years old, each month is about 1 billion years on this calendar. Our planet was formed in August (this would be beginning of the Precambrian eon on the stratigraphic chart). Dinosaurs roamed the Earth from about December 24 to 29, and it is only in the last few minutes of the year that we, humans, appear. This already tells us that a lot has happened on our planet before we appeared.

From: www.nobelprize.org (adapted from 'Cosmic Calendar' by Carl Sagan)

It is very interesting to consider that in the comparatively small amount of time that humans have existed, about 200,000 years, we have been able to transform the whole planet, develop incredible technology and even go beyond Earth. This has come at a price: pollution, higher levels of greenhouse gases in the atmosphere, deforestation, etc. Despite the evidence, many people refuse to believe that we are responsible for the present changes in the climate. As we will see in the following weeks, climate changes have happened before and will happen again - however we cannot say that our actions are insignificant to the global system, for smaller beings than us have changed the planet before.

An introduction

The Earth's climate changes. In its ~4.5 billion years, our planet has been covered in ice during Snowball Earth events more than half a billion years ago and it has been warmer than present day during the Pliocene, just to mention a couple examples.  
So, if our climate has changed so much before we even existed, why would we think that us, puny humans, are capable of causing increased average temperatures, melting of glaciers, rise of sea levels? How are we even sure it is not just natural variability, or caused by external factors? And most importantly, is everything going to freeze over the day after tomorrow? (Probably not).
I will be looking into different periods in the Earth's past and present to try to understand if we really could be behind all that is happening today. After all, the climate is changing, right?

http://xkcd.com/1321/