The Last 1,000 Years Summary The Medieval Warm Period The Little Ice Age Year Without a Summer Volcanoes and Climate Attribution of Climate Change Exam 2 on Wednesday!

Instrumental: Red Triangles Tree Ring: Brown Triangles Borehole: Black Circles Ice Core: Blue Stars Other: Purple Squares Locations of Proxy Records How might the locations of proxy records influence the climate reconstructions?

Medieval Warm Period Also known as the Medieval Climate Anomaly (MCA) ~1000 to 1200 AD Most pronounced in the North Atlantic region? Debate persists as to how significant this was globally. Quite evident in tropical South America, however.

Figure 7. Reconstructed tropical South American temperature anomalies (normalized to the 1961–1990AD average) for the last ∼ 1600 years (red curve, smoothed with a 39 ‐ year Gaussian filter). The shaded region envelops the ±2s uncertainty as derived from the validation period. Poor core quality precluded any chemical analysis for the time interval between 1580 and 1640 AD. Kellerhals et al. 2010

Viking Settlement of the North Atlantic

Little Ice Age ~1500 to 1850 AD Widespread global cooling, particularly in the North Atlantic region (Europe and eastern North America). Glacier advance in most of the world, best chronicled in Europe.

Discussion Question What are believed to be the two most important climate forcings during the period 1000 to 1850 AD?

Alan Robock Department of Environmental Sciences Tambora in 1815, together with an eruption from an unknown volcano in 1809, produced the “Year Without a Summer” (1816)

Tambora Largest volcanic eruption of modern times Largest volcanic eruption of modern times Located on Sumbawa Island along the east Sunda Arc Located on Sumbawa Island along the east Sunda Arc Heard up to 1400 km away! Heard up to 1400 km away! Ash remained in atmosphere for several years, leading to pronounced cooling and the “Year without a summer” in the northeastern U.S. in 1816 Ash remained in atmosphere for several years, leading to pronounced cooling and the “Year without a summer” in the northeastern U.S. in 1816

Year without a Summer

Alan Robock Department of Environmental Sciences Mann et al. (2000) Tambora in 1815, together with an eruption from an unknown volcano in 1809, produced the “Year Without a Summer” (1816)

Alan Robock Department of Environmental Sciences Tambora, 1815, produced the “Year Without a Summer” (1816) George Gordon, Lord Byron Percy Bysshe Shelley Mary Shelley

Alan Robock Department of Environmental Sciences Tambora, 1815, produced the “Year Without a Summer” (1816) “Darkness” by Byron I had a dream, which was not all a dream. The bright sun was extinguish'd, and the stars Did wander darkling in the eternal space, Rayless, and pathless, and the icy earth Swung blind and blackening in the moonless air; Morn came and went—and came, and brought no day, And men forgot their passions in the dread Of this their desolation; and all hearts Were chill'd into a selfish prayer for light: And they did live by watchfires—and the thrones, The palaces of crowned kings—the huts, The habitations of all things which dwell, Were burnt for beacons; cities were consumed, And men were gather'd round their blazing homes To look once more into each other's face;...

Discussion Question How do volcanoes influence climate? What are the physical processes?

Volcanoes and Climate Large volcanic eruptions inject sulphate aerosols into the troposphere and stratosphere, reducing the amount of solar radiation reaching the surface and therefore resulting in a negative radiative forcing.

Alan Robock Department of Environmental Sciences Explosive NET COOLING Stratospheric aerosols (Lifetime  1-3 years) Ash Effects on cirrus clouds absorption (IR) IR Heating emission IR Cooling More Downward IR Flux Less Upward IR Flux forward scatter Enhanced Diffuse Flux Reduced Direct Flux Less Total Solar Flux Heterogeneous  Less O 3 depletion Solar Heating H 2 S SO 2 NET HEATING Tropospheric aerosols (Lifetime  1-3 weeks) Quiescent SO 2  H 2 SO 4  H 2 SO 4 CO 2 H 2 O backscatter absorption (near IR) Solar Heating More Reflected Solar Flux Indirect Effects on Clouds

Alan Robock Department of Environmental Sciences Krakatau, 1883 The Loudest Explosion Ever Heard

Greatest impact from a volcanic eruption is within the first three years, with a peak cooling 1 year after.

Solar Forcing Historical records, paleo-archives, and model simulations suggest that the Medieval Warm Period was associated with positive solar irradiance forcing (relative to 1500 to 1899 mean values) while much of the Little Ice Age was associated with negative solar irradiance forcing.

Sunspots and Climate There is some evidence that sunspot activity positively correlates to surface temperature (e.g., greater sunspot activity is associated with higher temperatures). The Maunder Minimum of sunspot activity occurred at the heart of the Little Ice Age.

Discussion Question How/why does sunspot activity appear to be an important climate forcing?

Sunspots and Solar Irradiance During periods of low sunspot activity, total solar irradiance decreases. During years when sunspots are abundant, the amount of radiation emitted in solar flares is a maximum. Mechanisms operating within the sun simultaneously regulate both sunspots and net solar emissions. Recent observations from space suggests that the decrease in ultraviolet (UV) irradiance during low sunspot activity may be considerably greater than previously thought.

Sunspot Cycle 24 The current data for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 101. We are currently over six years into Cycle 24. The current predicted size makes this the smallest sunspot cycle in about 100 years.

Solar Forcing in the News 6/full/news html 67/n7316/pdf/nature09426.pdf news/science-at- nasa/2013/08jan_sunclimate/