Continental drift: effect on climate cycles

Even though it is only a tiny sliver of land, relative to the sizes of continents, the Isthmus of Panama had an enormous impact on Earth's climate and its environment. By shutting down the flow of water between the two oceans, the land bridge re-routed currents in both the Atlantic and Pacific Oceans. Atlantic currents were forced northward, and eventually settled into a new current pattern that we call the Gulf Stream today. With warm Caribbean waters flowing toward the northeast Atlantic, the climate of northwestern Europe grew warmer. (Winters there would be as much as 10 degrees C colder in winter without the transport of heat from the Gulf Stream.) The Atlantic, no longer mingling with the Pacific, also grew saltier.

http://earthobservatory.nasa.gov/IOTD/view.php?id=4073




It may seem fairly odd, but warming winters actually tends to lead to ice ages as it allows more snow to accumulate than would otherwise. The Laurentide and Euroasian sheets were huge and they only got that way by lots of precipitation.

Huybers (2007) supports the hypothesis of a gradual change in obliquity response with the identification of a linear, 2-Myr trend in the first moment of the power spectrum, M1, which is a weighted average of response frequency. He suggests that this trend toward lower frequency power results from long-term changes in the climate system such as cooling or ice sheet growth. However, we find two problems with this explanation of obliquity response change. First, a gradual shift in frequency response cannot explain why glacial cycles respond to obliquity modulation before 1.4 Myr but not after. (The Huybers model of obliquity pacing is insensitive to the modulation of obliquity forcing for all of the last 2Myr.) Second, in the LR04 stack the trend in M1 reverses at _2 Myr due to the presence of low frequency power from 3.5 to 2.5 Myr. Therefore, the shift to lower frequencies in d18O from 2 to 0 Myr may not be a simple response to long-term cooling or ice sheet growth. We suggest that climate records spanning at least the last 3Myr be used when interpretting the relationship between observed climate responses and the MPT.

However, our observations cannot determine whether late Pleistocene 100-kyr power results from groupings of 2–3 obliquity cycles.

Noticed that there were comments in Raymo's paper regarding Huybers:
And then concludes:

Albedos of typical materials in visible light range from up to 90% for fresh snow, to about 4% for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a blackbody. When seen from a distance, the ocean surface has a low albedo, as do most forests, while desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4.[6] The average albedo of the Earth is about 30%.[7] This is far higher than for the ocean primarily because of the contribution of clouds.

Human activities have changed the albedo (via forest clearance and farming, for example) of various areas around the globe. However, quantification of this effect on the global scale is difficult.

The classic example of albedo effect is the snow-temperature feedback. If a snow-covered area warms and the snow melts, the albedo decreases, more sunlight is absorbed, and the temperature tends to increase. The converse is true: if snow forms, a cooling cycle happens. The intensity of the albedo effect depends on the size of the change in albedo and the amount of insolation; for this reason it can be potentially very large in the tropics.

The Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments onboard the Terra and Aqua satellites. As the total amount of reflected radiation cannot be directly measured by satellite, a mathematical model of the BRDF is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance and bi-hemispherical reflectance.

A long-term cooling trend is often considered responsible for the change in glacial dynamics at the MPT. Mechanisms proposed for this transition include changes in sea ice formation (Tziperman and Gildor, 2003; Ashkenazy and Tziperman, 2004), a switch from terrestrial to marine ice margins in Antarctica (Raymo et al., 2006), a gradual increase in the insolation threshhold for ice-sheet ablation (Raymo, 1997; Paillard, 1998; Berger et al., 1999; Huybers, 2006), and changes in ice-sheet stability due to the gradual erosion of North American regolith (Clark and Pollard, 1998).

On the first page is the following paragraph:



Notice how Raymo is listed more than once!

So, take your pick; but nobody for the Ithmus of Panama.

For example, the tectonic closure of the Isthmus of Panama (Keigwin, 1982; Maier-Reimer et al., 1990), Tibetan uplift (Raymo et al., 1988; Rea et al., 1998), restriction of the Indonesian seaway (Cane and Molnar, 2001), shoaling of the thermocline (Philander and Fedorov, 2003), obliquity modulation (Haug and Tiedemann, 1998; Maslin et al., 1998), and changes in North Pacific stratification (Haug et al., 1999; Sigman et al., 2004) have all been proposed as mechanisms for the initiation of northern hemisphere glaciation.

Mammo; thanks for pointing that out.

So, a distinction is made between the initation of NH glaciation and the mid-Pleistocene
transition (MPT) from a 41K to 100K year cycles. Proposals for the MPT are more subtle.

Can't say that I really understand shoaling of the thermocline or changes in the North Pacific stratification. However, what impresses me is that all of these proposals may have played a part.

The development of this asymmetry is most likely due to a change in the internal dynamics of the climate system because asymmetry is not found in any orbital or insolation curves.

In this model ice volume from each hemisphere responds to local insolation, creating out-of-phase precession responses during the Pliocene and early Pleistocene. Before the appearance of large northern hemisphere ice sheets, the out-of-phase precession responses in the north and south nearly balanced out, leaving the d18O record dominated by in-phase obliquity responses. As northern ice volume increased, it began to outweigh the compensating southern response, and precession power in the global climate signal also increased. In contrast, the obliquity responses recorded by d18O are in-phase across hemispheres. Therefore, the sum of northern and southern obliquity responses could follow one continuous exponential trend despite possible changes in the ratio of response across hemispheres.

From Raymo: