Wednesday, May 23, 2018

Question About Human Vision and Missiles

Consider, say, a missile, 5 meters long and 1 meter in diameter. After it's launched, at what speeds could it be tracked by the human eye? What's the upper limit on that speed? Viz., when does the missile become effectively invisible?

(I know the diameter might not be realistic. I just wanted to make it an easy number.)

This is related to an article I'm working on.


Tuesday, May 22, 2018

James Chadwick Accidentally Went Into Physics

Image result for james chadwick neutron




"...James Chadwick, the discoverer of the neutron, had studied physics only because he was too shy to point out that he had mistakenly waited in the wrong line when matriculating."

From Warped Passages by Lisa Randall

Monday, May 21, 2018

Latest Ocean Heat Content Data

Global warming continues.

The data for ocean heat content for the first quarter of this year came out the other day for the global regions 0-700 meters and 0-2000 meters. They're warmer. Recall that changes in ocean heat content are the best way to detect the planet's energy imbalance -- over 90% of the heat trapped by our greenhouse gases finds it way into the ocean.

After some spreadsheet fun I get the following:


W=watts, J=joules, m=meters; yrs=years; Z=zetta=1021; T=tera=1012.

Note that the 0-700 m record is almost 5 times longer than the 0-2000 m record, so even though it's about 3 times smaller (in volume) it's not too surprising it's absorbed more heat over its record length. 

The uncertainties don't include autocorrelation -- the reality that one quarter isn't independent of the previous quarter, because a warm quarter is more likely to follow a warm quarter etc -- because I'm lazy and because I'm still not quite sure how to include it for the uncertainties of a 2nd-order polynomial fit. (Anyone know? Can you do it quick and dirty by using the effective sample size neff as in equation 9 of this document by Tom Wigley?)

Lots of graphs can be found here.



(In my calculations I've only included the Argo data for the 0-2000 m region, which starts in 2005, shown in red in this last graph.)

Friday, May 11, 2018

Crop Yields Under Global Warming

Up until now I thought that global warming would have a significant impact on crop yields, both in the US and elsewhere.

But I've gathered some numbers, and I'm so sure anymore -- at least for US farmers/farming corps.

My understanding was taken from papers like this one:
“For wheat, maize and barley, there is a clearly negative response of global yields to increased temperatures. Based on these sensitivities and observed climate trends, we estimate that warming since 1981 has resulted in annual combined losses of these three crops representing roughly 40 Mt or $5 billion per year, as of 2002.”
-- “Global scale climate–crop yield relationships and the impacts of recent warming," David B Lobell and Christopher B Field 2007 Environ. Res. Lett. 2 014002 doi:10.1088/1748-9326/2/1/014002
http://iopscience.iop.org/1748-9326/2/1/014002
and this one
“With a 1°C global temperature increase, global wheat yield is projected to decline between 4.1% and 6.4%. Projected relative temperature impacts from different methods were similar for major wheat-producing countries China, India, USA and France, but less so for Russia. Point-based and grid-based simulations, and to some extent the statistical regressions, were consistent in projecting that warmer regions are likely to suffer more yield loss with increasing temperature than cooler regions.”
- B. Liu et al, “Similar estimates of temperature impacts on global wheat yields by three independent methods, Nature Climate Change (2016) doi:10.1038/nclimate3115, http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3115.html 
But -- again, for the US -- these turn out to be quite small numbers, because the market sizes are so big and yields are increasing year-after-year.

So how do crop yields (again, US-only) vary with temperature? Here are some data from the National Climate Assessment (2014), Figure 18.3, p 421


These data are, admittedly, rather scattered and a straight-line trend will have errors. (I don't have the underlying data to calculate those here.) But by eye, I estimate the trends to be, for corn: -0.39 t/ha/°F, and, for soybeans: -0.08 t/ha/°F.

So what are the current yields, and how fast are they increasing? Here is the current yield and trend in corn yields, and the same for soybean yields.

So, plugging in the data, the trend in corn yields = 1.9 bu/acre/yr, and the trend in soybean yields = 0.5 bu/acre/yr. Relative to 2017, these are 1.1%/yr and 1.0%/yr, respectively.

From the same source, 2017 yields are, for corn: 176.6 bu/acre, and for soybeans, 49.1 bu/acre. "bu" is bushels.

I need to know the density of corn and of soybeans: 39.3680 bu/t and 39.7740 bu/t, respectively, from this source.

Translating into metric units: 2017 corn yield = 11.1 t/ha, 2017 soybean yield = 3.1 t/ha.

OK, now we can put things together.

Let's assume the surface warming trend is +0.20°C/decade. That's one degree Celsius in 50 years. (Results for other trends will scale linearly.)

Assuming the current trend in yields continues (iffy?), in 50 years (a long time, granted), yields will have increased by 170% (for corn), and 165% (for soybeans).

But in that time, yields will only decrease due to higher temperature by -6% (corn) and -5% (soybeans).

So agricultural technology will, even if trends continue at only a fraction of their current value, swamp any losses due to global warming.

And it won't take much increase in yields in developing countries for them to cancel out any loses due to higher temperatures, either.

--

Of course, there's no inherent reason to believe that yield increases will continue at their rate of the last 30 years for the next 50 years. Nor will warming stay linear, probably. And we'll need more food to feed ever more people, about 10 B by the middle of this century. And warming won't be limited to just one degree Celsius (we're already at that value anyway).

But I don't anymore see a big problem here. Am I missing something?

Sunday, April 01, 2018

Tuesday, March 27, 2018

Will the Falling Chinese Satellite Hit Someone?

As you probably know, the de-orbiting Chinese space station Tiangong-1 will soon fall to Earth.

This thing is 10.4 meters long and, on launch, weighted 8.5 tonnes.

What are the chances it will hit someone, or something important?

I don't know, and now that I'm reviewing this calculation I feel pretty sure I don't know, but here goes with my original idea.

The satellite will fall between the ± 43° latitude lines. But that's about all that seems known for sure right now.

That area constitutes 348 Mkm2, or 68% of Earth's surface area.

Much of this is ocean, but I don't know exactly how much. I'll assume, for better or worse, that it's the same percentage that the ocean makes up of Earth's total surface area, 70.8%, giving a potential land fall area of 102 Mkm2.

How many people are in this area? I don't know. All I know to do (without a lot of work) is assume it's the same percentage of the world population as is land, which, extrapolating this trend, was 7.46 B in 2017.

(Amazingly, 90% of the world's people live in the Northern hemisphere.)

When might a person be "hit" by a falling satellite? Let's say if it occurs within a 10 m radius of that person. Thus the danger zone around each person is 628 m2, and around all people living in the fall zone is (assuming independence) 3.20 Mkm2.

That's 3.1% of the satellite's potential landing zone. Small, but not tiny.

--

I suspect this number is an overestimate, because

(1) I've overestimated the number of people who live between -43 and +43 degrees latitude, since there's more ocean there than I've assumed, and
(2) I've assumed each person's "target zone" is independent of everyone else's (which is obviously not true in cities.)

Much of Europe and Russia lives above 43° N latitude, and all of Canada does -- and I do too, at 45-epsilon degrees. (I often drive past the a sign in North Salem announcing the 45° line, and based on my home's GPS coordinates I've calculated that I'm about 4.4 km south of this line, or at 44.960858° north latitude.)

So, there's a tiny chance of someone being hit by this satellite. 3% seems high -- but I doubt the chances are infinitesimal, even if I'm off by a factor of 10.

Still, good luck out there. Heads up.

Free, Online Climate Textbooks

I've recently come across a few free, online climate textbooks, and thought it worth mentioning here.

A great introduction to the basics:

Introduction to Climate Science, Andreas Schmittner, Oregon State University.

For more advanced students:

Introduction to climate dynamics and climate modeling, by Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (2008-2010).

I guess I can mention Pierrehumbert's textbook Principles of Planetary Climate, though I'm not sure it's on the Web legally. I bought the hardback version and have learned a great deal from it. I heartily recommend it.

Sunday, March 18, 2018

My Plastic Brain by Caroline Williams

From my mailbox:

My Plastic Brain: One Woman's Yearlong Journey to Discover If Science Can Improve Her Mind by Caroline Williams.

Amazon
Barnes and Noble

Author's Web site

From Kirkus Reviews:

"Readers hoping to improve their own cognitive abilities may feel a bit of a letdown by the author’s old-fashioned, down-to-earth advice: exercise your body, preferably outdoors, learn mindful meditation but also allow your mind to wander, engage in a mentally challenging hobby, and pick the skill you want to improve and practice it in real life."

Valar morghulis.