Late Paleocene Thermal maximum - A.K.A Global Warming (55.5 million years ago)

We left off 65 million years ago. What else could have happened between that time and now? The Cenozoic was no exception to the mood swings of our planet, though there is a cooling trend present after an initial warming.

Figure 1 shows us what was happening from the end of the Mesozoic to present day, though the Holocene, our present epoch, is almost indistinguishable since it started only 11700 years ago. 

Fig. 1: The δ¹⁸O record shows the temperature scale until around 35 million years ago, before the large scale glaciation of Antarctica and afterwards it reflects the changes in ice volume (towards the left means greater volume of ice). Zachos et al., 2001

The Late Paleocene Thermal maximum, around 55.5 million years ago, saw the highest temperatures in the Cenozoic, with deep sea temperatures rising by 6 °C in less than 10000 years (Zachos et al., 2001). As can be seen in the figure, it also coincided with a significant fall in the δ¹³C records. These characteristics suggest that an immense volume of ¹²C enriched carbon was added rapidly added to the ocean-atmosphere system. Dickens et al. (1997) argue that this ¹²C came from methane clathrates, since it is an immense reservoir (assuming it is similar to present day) that becomes unstable with higher temperatures. However, it remains unclear what could have triggered the initial dissociation of the methane clathrates – model results by Katz et al. (2001) were not conclusive if temperature rise was the initial trigger. Alternative proposed triggers are mechanical disruption of the clathrates, either by seafloor erosion caused by changes in ocean circulation or continental slope failure (Katz et al., 2001), or by bolide impact, though Bowen et al. (2015) did not find it consistent with the two release events of found in their study.

This event can be used as an analogue for present day anthropogenic warming and what we may be facing in the future. This time the trigger for releasing methane trapped in the clathrates is not unknown - the temperature is already rising.

Tweet of the week - Another record year?

A new year begins, so it is almost time to see how 2014 measures with previous years - did the warming trend continue?

Will 2014 be the warmest year on record? Watch this space #climate2014 pic.twitter.com/6kF4rZDs7M
— WMO | OMM (@WMOnews) January 2, 2015

To the surprise of all (... not really), the Earth continued warming. At the beginning of December, the WMO issued a press release that stated that, if 2014 continued the same trend in November and December as what was observed from January to October, it would be among the hottest - if not the hottest - year on record. Global air average temperature (Jan-Oct) was 0.57°C higher than the 1961-1990 reference period. Over land average air temperature was 0.86°C higher.

 

Average temperature over land will continue warming faster that over the sea. Figure 1 shows the change in surface temperature per degree Celsius of global mean change, as projected for the 21st century (IPCC, 2013).

Fig. 1: mean surface temperature increase per °C of global
mean temperature increase. Source: IPCC, AR5

 

This means that the average global temperature is just that, an average. A warming in 2°C is not distributed equally around the globe, but there can be cooler areas and other areas that are much warmer. While the globe may increase 1°C, the continents will be around 1.25-1.5°C warmer. The figure also shows that the warming will be even higher in the Arctic regions, because the localised ice-albedo feedback will allow an increase in absorbed radiation.

 

The final verdict on 2014 is still to come, but it will likely not be a surprise.

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!