Hook Echoes, a Strong Texas Tornado, and Softball-Sized Hail

April 26, 2015
2:34 pm

SEVERE WEATHER STATEMENT
NATIONAL WEATHER SERVICE FORT WORTH TX
436 PM CDT SUN APR 26 2015

TXC143-262200-
/O.CON.KFWD.TO.W.0007.000000T0000Z-150426T2200Z/
ERATH TX-
436 PM CDT SUN APR 26 2015

...A TORNADO WARNING REMAINS IN EFFECT UNTIL 500 PM CDT FOR SOUTHERN
ERATH COUNTY...

AT 436 PM CDT...A LARGE TORNADO WAS LOCATED JUST SOUTH OF
DUBLIN...MOVING EAST AT 25 MPH.

THIS IS A PARTICULARLY DANGEROUS SITUATION.

HAZARD...DAMAGING TORNADO.

SOURCE...RADAR INDICATED ROTATION.

IMPACT...YOU ARE IN A LIFE THREATENING SITUATION. FLYING DEBRIS MAY
         BE DEADLY TO THOSE CAUGHT WITHOUT SHELTER. MOBILE HOMES
         WILL BE DESTROYED. CONSIDERABLE DAMAGE TO
         HOMES...BUSINESSES AND VEHICLES IS LIKELY AND COMPLETE
         DESTRUCTION IS POSSIBLE.

THIS TORNADIC THUNDERSTORM WILL REMAIN OVER MAINLY RURAL AREAS OF
SOUTHERN ERATH COUNTY.

TORNADO...RADAR INDICATED
TORNADO DAMAGE THREAT...CONSIDERABLE
HAIL...4.25IN

_________________________________________________________________

The Earth has a lot of intense storm activity. Category five hurricanes can inundate cities and turn buildings to rubble. Lightning heats of the surrounding air to five times the surface of the sun, and creates a shockwave that can be heard for miles. Hailstones can weigh up to one kilogram, and cause billions of dollars of damage within less than an hour. But none of these can hold a candle to a category five tornado. Strong tornadoes are, without a doubt, the most terrifying and violent meteorological phenomena on a planet. Nothing else even comes close.

The U.S. doesn't get the strongest hurricanes (Philippines), the most lightning (Democratic Republic of the Congo), or the largest hailstones (Bangladesh), but we do get the most/strongest tornadoes. We get nearly all of our tornadoes east of the Rockies, but the majority of them occur in the Great Plains. In fact, so many of them occur there that the region has been given a name that has become ubiquitous throughout American culture. Tornado Alley.

I've been wondering when we'd see our next big tornado. Springtime is tornado time over Tornado Alley, but I just haven't seen that many tornado warnings popping up on the National Weather Service homepage this spring. I saw one today though, and when I took a look at the radar, I knew that this was not just some run-of-the-mill tornado. This was a damaging tornado, associated with an intense, supercell thunderstorm.

Fort Worth WSR-88D (Weather Service Radar 88D) image showing a strong tornadic supercell with a clear hook echo NE of Brownwood at 3:53 CDT. Retrieved from the Dallas/Fort Worth NWS Forecast Office.

I attached the picture above so that you could have a clear look at the actual radar imagery of the storm, but here's a picture below with the counties and warning zones highlighted. Yellow zones are severe thunderstorm warnings (hail of one inch in diameter or greater and wind speeds of 58 mph or greater), and red zones are tornado warnings.

Fort Worth WSR-88D (Weather Service Radar 88D) image showing a strong tornadic supercell with a clear hook echo NE of Brownwood at 3:53 CDT with counties and warning zones included. Retrieved from the Dallas/Fort Worth NWS Forecast Office.

So, how did I know that this storm was likely going to have a strong, damaging tornado? I knew so because the storm had a very clear hook echo.

Retrieved from NWS Storm Spotter Training

For a thunderstorm to be classified as a supercell, it can't just be massive, it has to rotate. Very few thunderstorms become supercell thunderstorms. The hook echo shown is produced by rain, hail, and in the case of a tornado, debris being wrapped around the supercell due to this rotation. One of the best examples of a hook echo was that of the 1999 F5 Bridge Creek-Moore Tornado, which was among the strongest every recorded. As you can see, the tornado was located right at the hook echo. Oh yeah, this storm produced the strongest winds ever measured on Earth at 301 mph. So yes, hook echoes aren't there just for show.

Retrieved from Wikipedia page on hook echoes

Thankfully, this storm that is currently going through will avoid the Dallas-Fort Worth metropolitan area, because if they were to receive 4.25 inch hail and a 'large, damaging' tornado, there could be over a billion dollars of damage and significant loss of life.

The latest update (530 PM CDT) says that although the tornado has ended, there is still 3-inch hail and 70 mph gusts with this storm. This is a very, very dangerous storm folks. If you want a tornado, you'll have to head down to Houston/Galveston. I'm not as worried about this one though, because the storm doesn't have a significant hook echo.

Houston WSR-88D (Weather Service Radar 88D) image showing a tornadic supercell headed ENE at 5:26 CDT with counties and warning zones included. Retrieved from the Houston NWS Forecast Office

Back here, there is a weak band of showers over the Olympics right now, so be prepared for the elements if you go outside tonight... you might get a tad wet. Don't forget to bring a towel!

UPDATE 8:00 PDT (10:00 CDT)

Look some of the latest pictures from the Dallas-Fort Worth scanner Twitter page. The below softball-sized hailstone was from the storm I talked above.

Photo credit: Zoey Rae James. Retrieved from Dallas-Fort Worth Scanner Twitter Page.

Also, they are now getting more tornadoes. The pictures below were also taken from their Twitter page. I bet you know where the tornado is! It looks like they do.

Radar showing hook echo, 9:39 pm CDT. Retrieved from Dallas-Fort Worth Scanner Twitter Page.

Radar showing hook echo, 9:45 pm CDT. Retrieved from Dallas-Fort Worth Scanner Twitter Page.

The National Weather Service is acting like they think this is a particularly strong tornado, perhaps even stronger than the one earlier today. As I said before, a strong hook echo is indicative of a strong tornado, and the hook echo on this storm is stunning.

THE NATIONAL WEATHER SERVICE IN FORT WORTH HAS ISSUED A

* TORNADO WARNING FOR...
  NORTHWESTERN HILL COUNTY IN CENTRAL TEXAS...
  SOUTHERN JOHNSON COUNTY IN NORTH CENTRAL TEXAS...

* UNTIL 1045 PM CDT

* AT 1010 PM CDT...A CONFIRMED LARGE AND EXTREMELY DANGEROUS TORNADO
  WAS LOCATED NEAR CLEBURNE STATE PARK...OR 9 MILES SOUTHWEST OF
  CLEBURNE...MOVING EAST AT 20 MPH.

  THIS IS A PARTICULARLY DANGEROUS SITUATION.

  HAZARD...DAMAGING TORNADO.

  SOURCE...WEATHER SPOTTERS CONFIRMED TORNADO.

  IMPACT...YOU ARE IN A LIFE THREATENING SITUATION. FLYING DEBRIS
           MAY BE DEADLY TO THOSE CAUGHT WITHOUT SHELTER. MOBILE
           HOMES WILL BE DESTROYED. CONSIDERABLE DAMAGE TO
           HOMES...BUSINESSES AND VEHICLES IS LIKELY AND COMPLETE
           DESTRUCTION IS POSSIBLE.

Charlie

An El Niño For California!

Wednesday, April 22, 2015

10:14 am

Last year, climatologists were warning us that we could have one of the biggest El Niños in history, perhaps one even rivaling the legendary El Niño of 1997-1998. Those forecasts never verified, but you wouldn’t know that by our weather this past winter. El Niño winters are generally warmer and drier than normal in the Pacific Northwest, and boy oh boy were we warm. While we had near-normal rainfall, most of this rainfall occurred in short, heavy spurts due to warm, subtropical systems soaking the area. All of this made for, as I’m sure you are well aware, a less-than-optimal ski season.

Precipitation and temperature departures from average. Credit: Climate Prediction Center's El Niño/Southern Oscillation (ENSO) Diagnostic Discussion

But this winter, it looks like a real El Niño may be in store. While I normally hate El Niños (I’m a skiing fanatic), I’m actually hoping a strong one arrives this year. Why, you might ask? 

One word. California.

Folsom Lake. Credit: California Department of Water Resources

The above picture shows pictures of Folsom Lake, a reservoir 25 miles northeast of Sacramento in Northern California. The picture on the left is from July 2011, and the picture on the right is from January 2014. During July 2011, the reservoir was at 97% total capacity and had 130% of its average capacity for that date. For January 2014, the reservoir was at 17% total capacity and had 35% of its average capacity for that date. I can't even tell if the reservoir reaches the dam in the foreground. The lack of water in California is frightening. Jerry Brown, the governor of California, has called for a 25% reduction of water usage in 2015 compared to 2013 levels, and numerous measures are being put in place to ration water. The latest, a "tiered" measure where the more water you use, the more you would pay per gallon, was recently ruled unconstitutional this past Monday. 

This is the kind of stuff that drives me crazy. There seems to be a common theme in American society of being unable to take action on a pressing environmental issue because it will violate some peoples’ rights. When a situation gets really, really serious, taking swift action is more important than finding a perfect solution. What's more important... having people who use large amounts of water pay unfair sums of money for using it (while farmers get off for free and Nestle (Arrowhead) illegally pumps their water from California’s national forests and sells it to drought-free states for a profit), or continuing to watch California's water supply dwindle? In my opinion, when there is a crisis, it is more important to get an imperfect solution in place that can be edited later than take no action at all. The lack of urgency towards mitigating environmental crises in politics confounds me. But that is a topic for a different blog.

So if California won't help themselves, will Mother Nature come to the rescue? I am cautiously optimistic that she will. But before we discuss how the current El Niño will unfold, let's take a look at the effects of past ones for the continental U.S. I hope you like pictures. :)

Typical climate patterns for the U.S. during El Niño winters. Credit: Climate Prediction Center/NCEP/NWS/The COMET Program

During El Niño years, the jet stream that comes off the Pacific and brings us our big storms tends to shift to the south. As a result, we tend to be warmer and drier, while California tends to be cooler and wetter. This pattern is not unique to the West Coast; during El Niño years, the entire northern and southern tiers of the U.S. follow a similar trend.

The maps below show the composite precipitation and temperature anomalies during El Niño years over the states. Of particular note is the sharp gradient in precipitation between the Cascades and the Sierra Nevada. With warmer and drier-than-normal conditions, the Cascades are often starved of snowfall, while the Sierra Nevada get absolutely crushed.

Composite Precipitation Anomalies (inches) during El Nino years. Credit: NOAA

Composite Temperature Anomalies (F) during El Niño years. Credit: NOAA

Now that we’ve got that covered, we’re ready to take a look at El Niño's current status. But before we do that, let’s quickly review our El Niño “regions.”

Credit: NOAA's National Climatic Data Center

El Niño occurs in the tropical Pacific, and there are different Niño "regions" that are used as an indicator of El Niño strength. They are all important, but Niño 3.4 is the one that is generally the most representative of an El Niño event. The graphs and maps below show the temperature anomalies in these regions over time. All of the pictures below were retrieved from the Climate Prediction Center's latest El Niño/Southern Oscillation (ENSO) diagnostic discussion.

SST anomalies from all of the Niño regions

SST anomalies over the tropical Pacific

As you can see, sea-surface-temperatures (SST) were above average for the entire winter, but for the tropical Pacific to be in an El Niño state, the SST must have a 3-month average of at least 0.5 °C above normal. We barely, barely made El Niño criteria this past winter, but as you can see, all of the Niño regions are warming rapidly, and we are now solidly in weak El Niño conditions.

Changes in ocean temperature at depth along the Equator

Warming is also very apparent at depth, as the positive temperature anomalies have increased throughout the water column in the central and eastern Pacific. 

The majority of models are on board for an El Niño event. There are three charts I like to look at: the "Probabilistic ENSO Forecast" from the Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI), an IRI/CPC compilation of Niño 3.4 SST predictions from different dynamical and statistical models around the world, and the Niño 3.4 SST predictions from the CPC’s own CFSv2 (Coupled Forecast System Model, version 2). As the Probabilistic ENSO Forecast chart below shows, there is a 60-70% chance of the El Niño lasting through 2015, and our chances of a La Niña developing (cooler-than-normal waters in the tropical Pacific) are small.

Graph showing the probabilities of an El Niño, Neutral, or La Niña event for the rest of 2015

The IRI/CPC’s dynamical/statistical model compilation shows all but one model forecasting above-average SST for the rest of the year, with the majority predicting El Niño conditions. The thicker lines are the averages of given models, and these also denote weak-to-moderate El Niño conditions for the rest of 2015.

Niño 3.4 SST anomalies as predicted by different dynamical and statistical models

The CFSv2 also shows a moderate El Niño developing. The solid black line is the measured SST in Niño 3.4 thus far, the colored lines are forecasts from different "ensemble" members; i.e. forecasts using slightly different initial conditions, and the dashed line is the average of these ensemble members. 

Niño 3.4 SST Anomalies from the CFSv2 model

For a more visual depiction of what this graph represents, take a look at the sequence of images below. These images are based on the above predictions from the CFSv2 model and show what the SST throughout the tropical Pacific are forecast to look like as we head into 2015.

Visual evolution of the El Niño as predicted by the CFSv2 model

In conclusion, an El Niño is already developing, and models are pretty confident that it will continue to strengthen and persist throughout 2015 into next winter. The effects of El Niño for North America are most pronounced during our winter, so if California gets buckets of rain this summer, don't drop down on your knees and praise El Niño just yet. On the other hand, if California is getting swamped in the middle of the winter, well, you'll know who to worship. 

Charlie

The Future of The Earth

Wednesday, April 8, 2015
12:07 pm

Edvard Munch's The Scream

As the picture above insinuates, the future is not good.

Now, when I'm talking about the future, I'm not talking about 100 years from now (although that's pretty bad too). I'm not even talking about 1,000 years from now. No, I'm talking in the realm of 7,000,000,000 (seven billion) years from now. When stars age, they gradually become bigger and brighter before finally becoming "red giants" where although they have the same mass, their volume is significantly larger. In seven billion years, the Sun may actually expand to a point where it engulfs the Earth. The Earth is currently 93 million miles, or 1 astronomical unit (AU), away from the sun (give or take 1.5 million miles depending on the season, as the Earth has an elliptical orbit and is further away from the sun during the Northern Hemisphere summer than the Northern Hemisphere winter). The diagram below has the sun with a diameter of 2 AU, meaning it has a radius of 1 AU and just barely engulfs the sun. Seeing as the sun is only 0.01 AU right now, this diagram shows that the sun as a red giant will be approximately 200 times wider, and using the formula for volume:

V

=

4

3

π

r

3

Where r = radius and pi (the Greek letter) is the ratio of a circle's circumference to its diameter (approximately 3.14), the volume of the sun as a red giant is predicted to be approximately 8 trillion times greater than the sun's current volume. They don't call it a giant for nothing. And the red color comes from the surface gradually cooling as it increases in volume.

Sun as a red giant compared to its current state. Credit: Oona Räisänen

But I'm getting ahead of myself here. Obviously, the Earth is kinda screwed if it becomes part of the sun. But we've got a number of hazards to watch out for before then.

Throughout the Quaternary Period (2.58 million years ago to the present), ice ages have been controlled by Milankovitch Cycles, which are changes in the Earth's orbit (eccentricity), axial tilt (obliquity), and amount of "wobbling" that the Earth's axis undergoes, kind of like a spinning top (precession). The exception, of course, is global warming due to increased greenhouse gases since the Industrial Revolution. However, regardless of any effects of from current anthropogenic global warming and greenhouse gas emissions, the Earth will likely enter its next ice age in 50,000 years (Berger and Loutre, 2002). If humans are still around by then and we haven't solved the carbon dioxide problem, an ice age may not occur. Additionally, we would be able to avoid an ice age altogether if we pumped enough carbon dioxide into the atmosphere to create an increased greenhouse effect to offset the decrease in solar radiation.

By 500,000 years, the Earth will likely have been hit by an asteroid 1km in diameter, assuming humans aren't still around and can't explode it in space or find a way to avoid it completely by delaying its impact (thermonuclear weapons are good for something!) (Hall and Ross, 1997), (Bostrom, 2002). By 1 million years, the Earth will have likely experienced a volcanic eruption similar in magnitude to the eruption of the Toba supervolcano 75,000 years ago, which was, as the picture below shows, much, much larger than the famed Mt. St. Helens eruption of 1980.

Photo Credit: USGS

Of course, now all that remains of Toba is a beautiful lake. However, it still has a magma system under the lake, so while it is a dormant volcano that is not erupting, it has the potential to erupt again, and thus is not extinct (Oregon State University). You wouldn't know it by looking from the tranquil picture of the crater lake below, but when Toba does erupt again, the results will be catastrophic, and the Earth will be plunged into a volcanic winter for approximately a decade with a period of cooler temperatures lasting for up to 1,000 years

Lake Toba. Photo Taken by A.M. & K.D. Hollitzer in 1996. Retrieved from Oregon State University Volcano World

NASA Landsat satellite image of Lake Toba. Credit: NASA

I've come across a lot of scary stuff in the Earth and space sciences, but the part I'm going to describe to you next may just be the scariest of all. And it's because it's our fault.

In 2 million years, coral reef ecosystems are expected to have finally completely recovered from ocean acidification. When I heard this, I was astounded. Two million years? That's an unfathomably long time from now. We're putting carbon dioxide into our atmosphere at an unprecedented rate, and we know that it is going to have drastic effects on sea life. Just look at the picture below! A note of caution... scientists aren't completely certain about how quickly global warming and ocean acidification will destroy coral reefs, so take the photograph with a grain of salt. Alarmists like to show this stuff for shock value. But the bottom line is that increased carbon dioxide will lead to increased ocean acidity and temperature, and this will decimate coral reefs around the world

The Effects of Ocean Acidification and Temperature Rise on Coral Reefs. Retrieved from Furman University's Ocean Acidification Page

The graph below is from the Intergovernmental Panel on Climate Change's (IPCC's) 5th Assessment report, and shows different carbon dioxide concentration scenarios. If we continue releasing CO2 at the rate we are currently doing so, we will reach 500 ppm in a couple decades. Just think... a couple centuries so start the dissolution of coral reefs, and 2 million years to rebuild them. I promise to have more blog posts on ocean acidification in the future, as I believe it is a subject that does not get as much attention as it should, mainly in the media but also in our educational institutions. In fact, ocean acidification was hardly discussed in any of my atmospheric science classes at the University of Washington, including those on climate.

CO2 Emission Scenarios from the IPCC 5th Assessment Report. Retrieved from Wasatch Weather Weenies Blog

A variety of interesting geological things will happen shortly after (in geological time). 10 million years from now, the East African Rift valley will finally spread open far enough to form a new ocean, (Haddock, 2008), and 50 million years from now, California will have slid up to Alaska and been subducted into the Aleutian Trench (Garrison, 2009). By 100 million years, we will have likely been hit by one of those huge asteroids like the one that killed the dinosaurs (Nelson, 2014). By 250 million years, the continents will likely have drifted together to form another supercontinent analogous to "Pangea," the supercontinent that existed 200-300 million years ago. However, this supercontinent will have likely broken up 450 million years from now (Williams and Nield, 2007).

But things really start to get interesting 600 million years from now. And by interesting, I mean depressing.

Dead Plants in Pots. Retrieved from Wikipedia.

As I stated before, stars gradually become brighter as they age. 600 million years from now, the sun is expected to become bright enough that there will be enough heat the atmosphere to evaporate a significant amount of water vapor and thereby cause heavy rain throughout the Earth. As this heavy rain falls, it combines with carbon dioxide in the atmosphere to make carbonic acid. This acid erodes the landscape through a process known as weathering. Eventually, this process, is expected to take the carbon dioxide levels in the atmosphere to a point below the vast majority of photosynthesizing organisms will die (Heath and Doyle, 2009). Normally, carbon dioxide would be continuously added to the atmosphere via volcanism, but so much water will have evaporated from the Earth's surface that rocks will become so hard that plate tectonics, the primary drivers for volcanism, will cease (O'Malley-James et al., 2012).

This process will continue, and by the next 200 million years, all photosynthetic life will be gone and multicellular life will die (Heath and Doyle, 2009).

One billion years from now, the sun will have become 10% more luminous. This doesn't sound like that much, but the average temperature of the Earth will now have risen to 116 degrees Fahrenheit (it's 61 degrees right now). And that's with next to no CO2 in the atmosphere! Why so hot? Well, this increase in solar radiation will also spur a "runaway greenhouse" where the oceans evaporate entirely (Schröder and Smith, 2008). Water vapor is the most important greenhouse gas in the atmosphere, and no single-celled organisms will be doubting that one billion years from now. In fact, eukaryotic life is expected to become extinct 1.3 billion years from now, with only prokaryotic life remaining (Franck et al., 2005). Prokaryotic life can be thought of "the most basic life there is."

And the news just keeps getting worse. By 2.3 billion years from now, the Earths outer core is predicted to freeze (Waszek et al., 2011). The outer core is currently liquid, and is responsible for creating the Earth's magnetic field. Without a magnetic field, our atmosphere would be blown away. By 2.8 billion years, our average surface temperature has risen to 296 Fahrenheit, enough to wipe out all life (O'Malley-James et al., 2012). By 3.5 billion years, Earth will be as hot as Venus (Hecht, 1994).

Five billion years from now, the sun will evolve into a red giant (Schroeder and Smith, 2008), and as previously stated, after 7 billion years, it will likely grow large enough to swallow the Earth. And that will be the end of our (not so blue) planet.

What the Earth might look like when the sun is a Red Giant. Retrieved from Wikipedia.

So what can humans do to stop this madness from happening?

Well, first of all, I believe we'll likely be extinct in the near future. Thermonuclear war is a very real possibility, and as crazy as it may sound, we have to be very careful or artificial intelligence may be able to develop some sort of sentience and destroy us. If we were alive 600 million years from now and wanted our dear plants to still be alive, we'd preferably see if we could move our orbit further away from the sun. But if we aren't around, this process is inevitable.

You think that's depressing? I don't think you want to know about the proposed "heat death" of the universe. I'll write about that soon.

References:

Berger, A & Loutre, MF (2002). "Climate: an exceptionally long interglacial ahead?". Science 297(5585): 1287–8. doi:10.1126/science.1076120.PMID 12193773.

Bostrom, Nick (March 2002). "Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards". Journal of Evolution and Technology 9 (1). Retrieved 10 September 2012.

C. D. Hall and I. M. Ross, "Dynamics and Control Problems in the Deflection of Near-Earth Objects,"Advances in the Astronautical Sciences, Astrodynamics 1997, Vol.97, Part I, 1997, pp.613–631.

 Franck, S.; Bounama, C.; Von Bloh, W. (November 2005). "Causes and timing of future biosphere extinction". Biogeosciences Discussions 2 (6): 1665–1679.Bibcode:2005BGD.....2.1665F. doi:10.5194/bgd-2-1665-2005. Retrieved 19 October2011.

Garrison, Tom (2009). Essentials of Oceanography (5 ed.). Brooks/Cole. p. 62.

Haddok, Eitan (29 September 2008). "Birth of an Ocean: The Evolution of Ethiopia's Afar Depression". Scientific American. Retrieved 27 December 2010.

Heath, Martin J.; Doyle, Laurance R. (2009). "Circumstellar Habitable Zones to Ecodynamic Domains: A Preliminary Review and Suggested Future Directions".arXiv:0912.2482.

Hecht, Jeff (2 April 1994). "Science: Fiery Future for Planet Earth". New Scientist (subscription required) (1919). p. 14.

Lunine, J. I. (2009), "Titan as an analog of Earth’s past and future", European Physical Journal Conferences 1: 267–274, doi:10.1140/epjconf/e2009-00926-7.

Nelson, Stephen A. "Meteorites, Impacts, and Mass Extinction". Tulane University.

O'Malley-James, Jack T.; Greaves, Jane S.; Raven, John A.; Cockell, Charles S. (2012)."Swansong Biospheres: Refuges for life and novel microbial biospheres on terrestrial planets near the end of their habitable lifetimes". arxiv.org. Retrieved 2012-11-01.

Schroder, K. P.; Connon Smith, Robert (2008). "Distant Future of the Sun and Earth Revisited". Monthly Notices of the Royal Astronomical Society 386 (1): 155–163.arXiv:0801.4031. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x.

"Toba, Sumatra, Indonesia". Oregon State University.]

Waszek, Lauren; Irving, Jessica; Deuss, Arwen (20 February 2011). "Reconciling the Hemispherical Structure of Earth's Inner Core With its Super-Rotation". Nature Geoscience 4(4): 264–267. Bibcode:2011NatGe...4..264W. doi:10.1038/ngeo1083.

"When humans faced extinction". BBC. 2003-06-09.

Williams, Caroline; Nield, Ted (20 October 2007). "Pangaea, the comeback". New Scientist.