Yesterday was the 10th anniversary of the passing of the great astronomer/exobiologist/science popularizer Carl Sagan, and there's a world-wide blog-a-thon to celebrate.  Here's my entry.

Until I was told of the 10th anniversary, I hadn't realize just how powerful an influence Carl Sagan has had on my life and my intellectual development. 

Back in 1976, when I was 11, the most exciting thing in all possible worlds was about to happen.  On July 4, our nation's bicentennial, we were going to land an unmanned probe called Viking I on Mars.  It was going to look for life, the first real attempt to look for living organisms on another planet.  And Carl Sagan was there to explain all the tricky science bits, appearing over and over on CBS News and PBS.  The landing was slightly delayed, because they wanted to find a spot which was smooth so the thing wouldn't crash - and eventually set down on July 20 - a different anniversary, that of Neil Armstrong and Buzz Aldrin walking on the moon 7 years before.  I remember waking up as early as I could that day - 8 am, which is still early for me - and rushing down the stairs to turn on the TV.  I knew Viking was landing that day, but I didn't know when, and I didn't want to miss it.  Minutes after I had the TV on, they showed the first image taken from the surface of Mars - a shot of one of the Viking craft's own round feet.  Shortly thereafter, we had the first shots of the Martian landscape - with a blue sky, which meant water!  Of course we later realized that the colors were calibrated wrong and the skies were actually pink with iron oxide (rust).  We didn't conclusively find life - some people are still arguing about what exactly the 1976-technology life experiments showed.  But Carl Sagan was there to explain it all to us.  It's sad that he's not around anymore to discuss the implications of the more recent findings that Mars used to have water on it - and lots of it.

But Carl was there when I was in high school, too, with his glossy "Cosmos" TV show.  Every time I hear the phrase "Doppler shift" I still think of the footage of the guy riding around in the blue and red scooter from that show.  (The Doppler shift is when waves - like light or sound - change frequency as they're moving away or toward you.  This is why a train whistle changes pitch when the train passes.)  And his show introduced me to the great Dutch astronomer-mathematician Christiaan Huygens (1629–1695).  Huygens helped develop calculus and the idea that light was a wave, and he discovered that the rings of Saturn were separate from the planet, and he invented the pendulum clock.  The probe that landed on the Saturnian moon Titan two years ago was named after him.  But a remarkable thing about him was that he threw outrageous parties.  Not parties noted for their drunken carousing, but noted for their intellectual stimulation - DeCartes would show up and talk about the new math.  Rembrandt would display his latest doodles.  Van Leeuwenhoek would pass around his new microscopes like they were party favors.  And Huygens himself would entertain people on the harpsichord.  And this inspired me to throw my own "Christiaan Huygens" parties, where people would play music or recite poetry and show artwork.  And all because, in one tiny segment in "Cosmos," Carl Sagan had mentioned Huygens' parties in passing.

I want to close with a really significant contribution Sagan and his associates made to the intellectual landscape, which has important reverberations to this day, and possibly for tomorrow as well.

In 1983, Sagan and his friends published the TTAPS report on the potentially disastrous global environmental effects of a nuclear war.  The authors were R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Sagan (hence, TTAPS).  One of the main concerns about nuclear war had been the fallout (in the 1959 film "On the Beach" Australians sadly await the coming radioactive clouds which have already wiped out all other human life on the planet).  And in the fifties radiation was the creator of giant ants and monstrous lizards that threatened our homes.  The idea of "nuclear winter" also took into account the fact that cities, attacked by nuclear weapons, would burn, the atomic fireball fed by broken gas mains and all the flammable wood and plastics that fill modern cities.  The radioactive fallout might be rather localized, but the smoke from hundreds of nuclear fireballs all over America and the Soviet Union would rise and spread over the globe, covering most of it (except possibly South America, South Africa and Australia).  The sun would be blotted out and temperatures drop - the biggest drops (of 36 F) in the American midwest and Soviet Central Asia.  For the next year or two or three, sunlight levels would drop, rainfall patterns would shift, the growing season would be very short - crops would fail, multitudes would starve, governments and other institutions which (more or less) preserve order would collapse.  

Luckily, we never blew ourselves up.

However...  in the last couple years, there have been growing concerns about nuclear weapons being fired at a small scale.  What if terrorists get their hands on a weapon?  What if North Korea developes its bombs further and then uses them?  What if Pakistan and India come to blows?  Or if Iran decides to make good on its threat to wipe out Israel?  OB Toon and RP Turco (the "TT" in "TTAPS") recently published, with others, articles about the global effects of a "small" nuclear exchange.  Or even a single detonation.  "A single 'small' nuclear detonation in an urban center could lead to more fatalities, in some cases by orders of magnitude, than have occurred in the major historical conflicts of many countries. ... [S]moke from urban firestorms in a regional war would rise into the upper troposphere due to pyroconvection ... and then might induce significant climactic anomalies on global scales.  We also anticipate substantial perturbations of global ozone." (Toon et al. 2006 Atmos. Chem. Phys. Disc. 6:11745-11816).  

The term "nuclear winter" was actually coined by RP Turco in the original 1983 TTAPS report and first appeared in the title of their article in the journal "Science."  This was a last-minute change after a final review of the paper by a NASA bureaucrat raised concerns when the original phraseology "nuclear war" and "nuclear weapons" appeared in the title.  (See: A Path Where No Man Thought, Sagan and Turco, 1990, Random House, p. 465).  But it was Sagan who popularized the phrase "nuclear winter," and his words which haunt us and warn us today about which path the world should take.