We are professors of Atmospheric & Oceanic Sciences and Volcanology at McGill University, currently teaching an online course on Natural Disasters. Ask Us Anything!

Is the ocean acidification rate (= atmospheric CO2 dissolves in the seawater) slowing or accelerating? How long can it continue without becoming a major problem to humanity?

Ocean acidification is already a significant problem to humanity. It appears to be accelerating, particularly in nutrient-rich regions, particularly coastal regions, where, for example, the US Pacific Northwest oyster industry is vulnerable to its effects.

What would happen if hurricane hit an erupting volcano?

Well, the heavy rains associated with a hurricane, when pouring onto relatively unstable landmass of a volcano, erupting or not, can have cataclysmic impacts. An excellent example of this interaction during Hurricane Mitch in October 1998, when the Casita Volcano flank failed, and a lahar developed from the excessive rains. This lahar was responsible for many fatalities when two communities were destroyed.

Hello Professors

Can you give us 1) some thoughts on the El Niño phenomenon this year? And 2) your opinions on the use of data (Data Science, Big Data) on the mitigation and prevention of natural disasters

(I'm currently living in Potosí, Bolivia where in previous Niños we have heavy rainfall but this year a severe drought instead)

edit: format of the questions

This is an excellent point. To start, it is true that the current El Nino is among the top three strongest during the past 60 years. However, the particular global teleconnections are not always as we might expect from the textbooks. There are definitely distinct 'flavors' of each El Nino, with attendant variability that you are observing in Bolivia. Second, depending on the particular natural disaster, the use of large datasets and sophisticated computer models can be very helpful in mitigating and preventing negative impacts of hurricanes.

Is the sort of preservation we've seen at Pompeii unique? If so, why? It seems like we should have numerous examples of this sort of thing around volcanoes across the world.

It is certainly the best known. It is partly because the houses helped preserve the people and artefacts as the houses filled up with ash and pumice. I am sure there are other examples, so I would not think this is unique.

Is ocean acidification real and how can we participate in stopping it?

Yes, it is real. We can participate in halting this acidification by reducing our CO2 emissions. A substantial portion, up to 40%, of the CO2 from human activity may dissolve into our waterways, decreasing the pH.

Hi, thanks for doing this AMA. As I've understood CFC:s or Freons are so stable that they eventually end up in the Stratosphere, but what makes the levels decline in other words what makes CFC go away from the Stratosphere?

Well, the residence time of CFCs in the stratosphere is several years. The most efficient means of making them go away is through the episodic mixing into the troposphere, where they may be precipitated out.

Hi! With Cop - 21 proposing limiting temperature rise to 1.5 degrees do you think geoengineering is the only realistic way of achieving this goal? Also which method of CCS carries the most potential? Personally I favour artificial silicates through soil engineering. Thank you in advance.

The difficulty with geo-engineering strategies is that there may well be unintended negative consequences that may overwhelm the problem that you are trying to solve. For example, cooling certain regions of the world could have consequences globally, such as possibly eliminating the monsoon season in regions that are dependent on these rains for their food supplies. Really, no one has devised a cost-effective means of CCS. It is still an area of active research.

Let's imagine the scenario of the thinning polar ice cap being destroyed in a single event / few events (e.g. by storms or high seas). Is this realistic? Would this accelerate climate change?

Well, it is possible for a very thin ice layer over the Arctic Ocean to be mixed away by a strong storm, as may be the case during the months of August-September. The impacts of such an event may be felt in future storms during the next several weeks. The longer term climate change associated with reductions in sea ice are related to a much more complicated set of processes (such as large-scale atmospheric heating, large-scale ocean and atmospheric currents, etc.), and are not expecially affected by a single storm event.

What causes some hurricanes to weaken a bit before making landfall? Hurricane Katrina, for example

Hurricanes may weaken over the ocean for any one of several reasons. These reasons include, but are not limited to 1) passage over cooler water, 2) the environmental wind shear increasing, 3) and impacting drier air.

Hi, I'm from Mexico. What exactly happened with hurricane Patricia last year?

Hurricane Patricia was the strongest hurricane in the western hemisphere in terms of its maximum wind speed at 200 miles per hour, and the lowest central pressure at 879 hPa. Patricia was one of a record-shattering nine major hurricanes during the 2015 eastern North Pacific hurricane season.

Hello sirs. I'm Jessi by the way. I wasn't sure of I would be home for this so I posted this in the discussion board as well, but thought it couldn't hurt to ask here too.

In the 7th video you spoke of ash being heavy when it got wet. Heavy enough to tip an air plane. Why is this?

One of the main hazards of volcanic ashfall is when it accumulates to thicknesses where roofs can collapse. Because ash is commonly fine-grained particles (much finer than sand sometimes), the ash can absorb and hold a lot of water, making the ash much heavier than in a dry state. So if ash is falling and it is also raining, the roof collapse hazard can be very high, and people need to periodically go on top of their roofs and shovel off the ash. Of course, this action of shovelling is also hazardous, so people need to be extremely careful when doing so.

So in terms of tipping planes, the weight of ash, especially on the wing and tail surfaces, can tip planes on the ground, especially if the ash is wet. This happened in the case of the Mt Pinatubo eruption in the Philippines in 1991.

Dear Professors,

Do you think we can halt climate change with our current capitalist economic system?

Well, a key component in halting our carbon emissions is to reduce our reliance on fossil fuels. Currently, much economic success is directly due to fossil fuel based economies. Nevertheless, it is quite possible to redirect our economies towards more sustainable energy sources, such as solar, wind, or nuclear power.

It seems that flash floods occur more recently now (due to climate change?!) and I would call them a natural disaster too. What can we do to prevent flash floods from happening? We cannot stop heavy precipitation...

Yes, you are correct on both counts. Flash floods are probably occurring more frequently, and it may be at least partially associated with climate change. They are definitely a natural disaster. The primary means of protecting ourselves against the impacts of flash floods is to 1) not live in areas that are susceptible to flash floods, such as in canyons, and 2) have a feasible plan for a quick, safe, evacuation in case you are in harms way of such a flood. The primary contributor to increases in flash flooding impacts may be simply that people are placing themselves in vulnerable regions, such as canyons by either residing in these areas, or hiking in such areas, without consulting weather forecasts. The primary contributor to increasing flash floods from a climate change perspective is that the atmospheric environment is becoming both warmer and more moist, and this fact enhances the likelihood of heavy rains, and attendant flash floods, to occur.