I believe that in “A Hacker's Guide to Planet Cooling” [Science Agenda], the editors support the wrong approach to fixing our climate woes when they argue for experiments involving geoengineering methods such as fertilizing the ocean with iron to stimulate plankton growth.

Iron fertilization seems a bad idea: it alters the food chain and water chemistry, whereas the sequestration of carbon dioxide is only temporary. An “all hands on deck” approach will most certainly be warranted, but I find geoengineering and similar heavy-handed or fanciful solutions to climate change counterintuitive. Humanity has always busied itself resolving rather than avoiding problems.

Maybe an equal or greater focus should be on limiting population growth, reducing consumption, fostering clean energy, increasing science education, electing scientifically literate politicians, living closer to the earth and fostering an environmental value set in our society.

Fort Collins, Colo.

Environmentalists opposed to geoengineering our planet's atmosphere should welcome a recent paper in Science that shows that iron fertilization is both natural and effective.

The March 21, 2014, study by Alfredo Martínez-García of the Swiss Federal Institute of Technology in Zurich and his colleagues shows that during the last ice age, iron-rich dust blowing off the Southern Hemisphere's continents maintained high plankton levels in the Southern Ocean, which explains a large reduction of atmospheric carbon dioxide.

Peppermint Grove Beach, Western Australia


Better Than Earth,” René Heller's article on habitable super-Earths—planets up to 10 Earth masses larger than our planet with radii between Earth and Neptune—states that a planet with a higher mass, and therefore a flatter surface, and oceans could be an “archipelago world” of island chains more conducive to biodiversity. But this flatter solid surface, having less distance between its highest and lowest points, would also reduce how much water the planet could harbor without becoming a total water world devoid of any exposed land. Therefore, such a planet's biodiversity could be more sensitive than Earth to its ocean volume and to its climate.

via e-mail

HELLER REPLIES: Indeed, the flatter a planet with a given radius, the less water it could have without becoming an ocean world. In an extreme case of a perfectly smooth planet with high surface gravity, a single glass of water could cover the whole world with a nanometer-deep “ocean.”

Even so, by virtue of their larger size, super-Earths could support both large oceans and flat topographies. A 1.4-Earth-radii planet with an average ocean depth of two kilometers would have a total ocean volume of two billion cubic kilometers—nearly twice that of Earth's oceans—even with flattened landmasses. Archipelago worlds could have larger ocean volumes than Earth's.


Eddy Nahmias's article “Why We Have Free Will” is oddly devoid of support for the existence of free will. Instead Nahmias argues for a distinction between conscious and unconscious thought and then conflates conscious thought with free will.

If you produced an exact physical duplicate of a time slice of the universe, then everything that is true about what is going on in that first chunk would be true of the duplicate chunk. This concept, rather than any variation on the interaction between conscious and unconscious thought, is what makes free will an illusion.

University of Toronto

Nahmias argues that neuroscientific findings are consistent with our having free will. Yet “free will” is a vague and ambiguous term that is used in diverse ways in law, theology, artificial intelligence and philosophy. And in using it, Nahmias silently shifts around from a concept of self-control to one of consciousness to one of responsibility to one that depends on an opinion poll. What's more, he neglects to address the relativity of free will: our will is free from certain forces and not free from others, and it is free to do certain things and not others.

McAllen, Tex.

I would suggest the conscious mind is much like an executive of a large organization, delegating the power to make decisions and take action to subordinate “subconscious” neural modules that report back to the executive.

Suppose I tell a student to perform an experiment and bring me the results. The actual results may be known to the student days before they are finally reported to me, but the decision to initiate said experiment was still my decision.

San Antonio, Tex.


In reporting on a surface-air-pressure diving suit called the Exosuit in the article “In Search of Sunken Treasure,” Philip J. Hilts leaves the reader to view it as new, golly-gee technology.

I am surprised that Hilts does not recognize the JIM suit, an atmospheric-pressure diving suit that was deployed more than 40 years ago and perhaps best known to the general public for its appearance in the James Bond 1981 movie For Your Eyes Only. Several other suits followed it.

Houston, Tex.

HILTS REPLIES: The Exosuit is the latest and most sophisticated in a long line of atmospheric diving suits made to allow people to dive and do work in the deep while at surface pressure. The first one was built in 1715—out of oak. It was essentially a barrel with armholes that were cuffed, and it went to 60 feet.

Phil Nuytten, creator of Nuytco Research's Exosuit, has been developing such suits since the 1970s. He says the difference between the JIM suit and the Exosuit is enormous. For most of its life, the JIM suit had no thrusters to make it mobile. It also had joints on the arms and legs that were difficult to move in deep water, and the electronics were very primitive.

The Exosuit has gyros and sensors so the pilot and the people onboard the surface ship can know its exact position, and it has thrusters that can be controlled by the pilot or from the ship. The Exosuit has many other advances as well, and Nuytten says there are more developments coming, such as a model that swims with fins, completely independent of the surface boat.


The “Plant Vasculature” section of “Living Large,” by Kate Wong, erroneously describes the accompanying micrograph as a section of the stem of a Ranunculus plant, with chloroplasts in white. It instead shows a root, and the white objects are food-storing plastids.


Clara Moskowitz's review of “Finding Zero: A Mathematician's Odyssey to Uncover the Origins of Numbers,” by Amir D. Aczel [Recommended], referred to mathematics depending on “the numerals 0 through 10.” It should have referred to “0 through 9.”