Water Unit Introduction

Outline:• Unique chemistry• Distribution on planet - changes with time • Hydrologic cycle - redistribution, climate effects• Ocean circulation - basic wind & thermohaline

Chemically unique because it:• exists in all three phases at atmospheric temp & pressure (high melting

and boiling points for size & bond type- ammonia-NH3-mass 17, MP -78, BP 33)

and is the only substance that does this!• has covalent bonds for molecule, hydrogen bonds for liquid/solid

phase (10x stronger than usual inter-molecular bonds, but weaker than metal or ionic bond)

Chemically and physically unique because it has:• high heat capacity - 4.2 J/g/K- i.e., takes more

energy to raise temp, therefore stores more, important for spreading planet’s heat

• high heat of fusion (solid <> liquid ~ 335 J/g/ºK) and vaporization (liquid <>gas ~ 2400 J/g/ºK) so more effectively transfers heat when changing phase in atmosphere/ocean

• conducts heat more readily as a liquid

Water is chemically and biologically unique because it: • has high surface tension so forms drops & helps make good cell

membranes• can dissolve other compounds very easily (most common solute),

so can transport many things, e.g. NaCl, nutrients• mediates or facilitates most chemical reactions in living systems

(e.g., takes water to photosynthesize) - neutral pH

Water is physically unique because it is less dense as a solid (ice) than as a liquid.

Distribution on Earth“Water, water everywhere, nor

any drop to drink.” – Coleridge, Rime of the Ancient

Mariner

Hydrologic Cycle (in 103 km3)

Global warmingGlobal warming(temperature increase)(temperature increase)

Effects of global warming on water cycleEffects of global warming on water cycle

Speeds up globalSpeeds up globalwater cyclewater cycle

More extreme weather eventsMore extreme weather events• DroughtsDroughts• StormsStorms• FloodsFloods

Projected Changes in Annual Precipitation for the 2050sProjected Changes in Annual Precipitation for the 2050s

Source: The Met Office. Hadley Center for Climate Prediction and Research

The projected change is compared to the present day with a ~1% increase per year in equivalent COThe projected change is compared to the present day with a ~1% increase per year in equivalent CO 22

GreenGreen •• = increasing, Brown= increasing, Brown •• = decreasing= decreasing

Global Precipitation Trends Global Precipitation Trends (% per decade) (% per decade) 1900 - 19941900 - 1994

Source: Watson 2001

U.S. Precipitation Trends: 1901 to 1998U.S. Precipitation Trends: 1901 to 1998

Source: National Climatic Data Center/NESDIS/NOAA

GreenGreen •• = increasing, Brown= increasing, Brown •• = decreasing= decreasingAll stations/trends displayed regardless of statistical significanceAll stations/trends displayed regardless of statistical significance

Extreme Precipitation Events in the U.S.Extreme Precipitation Events in the U.S.

Source: Karl, et.al. 1996.