The pole star and Arctic climate change
Hello Weather Underground bloggers, Aaron here. Dr. Masters is on vacation so I'll be posting a series of vacation blogs for him. This is the first.
In my previous blog, I mentioned how the Arctic Climate Impact Assessment (ACIA), a 2004 study of Arctic climate compiled by 300 scientists over three years, found that the recent increase in Arctic temperatures was probably due to human-emitted greenhouse gases. Greenhouse skeptics attacked the ACIA report and its conclusion, pointing out that the Arctic was much warmer than today during the period 4,000 - 7,000 years ago. For example, Dr. Patrick Michaels said in a 2004 interview with CNS news, a conservative Internet news service:"It was warmer 4 to 7,000 years ago [in the Arctic.] Every climatologist knows that. I saw no mention of that in the Arctic report," Michaels said. He added that the past warming of the Arctic couldn't possibly be blamed on greenhouse gas emissions since it occurred long before the industrial era.It is true that Arctic summers were warmer during the period 4,000 - 7,000 years ago. The mean July temperature along the northern coastline of Russia may have been 2.5 to 7.0 �C warmer than present, and Scandinavian summer temperatures were 1.5 to 2 �C higher than at present. The warming was caused by changes in the amount of sunlight the north pole gets in summer due to variations in the Earth's orbit.
Figure 1. Summertime temperatures in the Arctic during the Mid-Holocene Warm Period (about 6,000 years ago), compared to today's temperatures. Image credit: NOAA.
Earth's orbital variations
Earth's orbit is not perfectly circular, which means that we are closest to the sun in December and farthest in July. Thus, the Southern Hemisphere gets more sunlight in their summer (December) than the Northern Hemisphere does in their summer (July). The Earth's current 23.5 degree tilt keeps the north pole pointed towards the star Polaris, which results in the north pole being pointed at the sun in December (Northern Hemisphere winter). The Earth slowly wobbles around its axis with a period of 23,000 years a href=(precession) so that the pole star gradually changes with time. During the period 4,000 - 7,000 years ago, the pole star was Thuban in the constellation Draco, and the north pole was pointed at the sun during Northern Hemisphere summer. This resulted in much warmer summer temperatures in the Arctic, since there was more summer sunlight. Conversely, winter temperatures were colder, since the Earth was farther from the sun during the Northern Hemisphere winter. The Arctic will again be much warmer in summer 16,000 to 19,000 years from now when the cycle repeats and Thuban is once more the pole star. The reason the ACIA failed to mention this climate period is that the average temperature in the Arctic remained about the same during the period 4,000 - 7,000 years ago; it was just the summers that warmed. The warming since 1980 and the 1930s were warmings over all seasons, so it was misleading for the climate skeptics to compare Arctic temperatures 4,000 - 7,000 years ago with these modern warmings.
This is the second in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday. Next blog: Why the Arctic sea ice is shrinking.
Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.
Jeff Masters
Reader Comments
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Tempête Tropicale Modérée Favio
35 knots (10 min), 995 hPa
If Neutral Conditions are in effect then it really it won't be a very active season.
The section "Earth's orbital variations" is not very
accurate. I suppose it meant to say that due to precession
the length of summer was longer 4,000 - 7,000 years ago
(Earth closest to the sun varies wrt. the seasons). Or was
the inclination much different 4,000 - 7,000 years ago?.
BTW: How can a difference of few days in the length of the
summer make a difference of 1.5 ºC to 7.0 ºC?
"the north pole being pointed at the sun in December"
It is pointed ***away*** from the sun in December.
"north pole was pointed at the sun during Northern
Hemisphere summer"
This is always be the case, by definition. Otherwise it
would not be summer.
"closest to the sun in December and farthest in July."
The Earth is (currently) closest to the Sun on 3rd January and farthest
on 7th July.
It does not matter where the north pole is pointed (the Sun
at e.g. summer solstice will just appear to be in different
constellation). The inclination of the rotation axis (to
ecliptica) can be exactly the same for the entire precession
cycle.
"The Earth's current 23.5 degree tilt keeps the north pole pointed towards the star Polaris, which results in the north pole being pointed at the sun in December (Northern Hemisphere winter). The Earth slowly wobbles around its axis with a period of 23,000 years a (precession) so that the pole star gradually changes with time. During the period 4,000 - 7,000 years ago, the pole star was Thuban in the constellation Draco, and the north pole was pointed at the sun during Northern Hemisphere summer."
read as
The Earth's current 23.5 degree tilt keeps the north pole pointed towards the star Polaris, which results in the north pole leaning away from the sun (Northern Hemisphere winter) while the earth is closest to the sun in its orbit (aphelion). The Earth slowly wobbles around its axis with a period of 23,000 years a href=(precession) so that the pole star gradually changes with time. During the period 4,000 - 7,000 years ago, the pole star was Thuban in the constellation Draco, and the north pole was leaning toward the sun (Northern Hemisphere summer) while the earth was closest to the sun in its orbit (aphelion).
it would be a more precise statement but let's not lose sight of Dr. Master's point.
4,000 - 7,000 years ago Arctic summers were warmer and Arctic winters were colder. Average temperatures have remained about the same. It was misleading (I'd call it just plain deceit) for Dr. Patrick Michaels to mention only the ancient warmer summers.
It has to do, I think, with whether the earth is closer to perihelion or aphelion in a given season, i.e. winter aphelion=greater seasonal variation.
Sounds right. Northern winter would start in what is now July.
The figure above illustrates the effects of axial precession on the northern hemisphere seasons, relative to perihelion and aphelion. The precession of the equinoxes contributes to periodic climate change, and is known as the Milankovitch cycle.
Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the orbital eccentricity is extreme, the seasons on the far side of the orbit may be substantially longer in duration. Today, in the northern hemisphere, when fall and winter occur at closest approach, the earth is moving at its maximum velocity and therefore, fall and winter are slightly shorter than spring and summer. Today, the northern hemisphere summer is 4.66 days longer than its associated winter and spring is 2.9 days longer than fall. Axial precession slowly changes the place in the Earth's orbit where the solstices and equinoxes occur. See tropical year for a more extensive treatment and numerical values. Over the next 10,000 years, northern hemisphere winters will become gradually longer and northern hemisphere summers will become shorter, eventually creating conditions believed to be favorable for triggering the next ice age.
credit NOAA
They need bigger dots for their key.
GLOBAL AVERAGE TEMPERATURE FOR JANUARY HIGHEST ON RECORD, U.S. TEMPERATURE NEAR AVERAGE FOR MONTH
(global highlights below)
Feb. 16, 2007 — The combined global land and ocean surface temperature was the highest for any January on record, according to scientists at the NOAA National Climatic Data Center in Asheville, N.C. The most unusually warm conditions were in the mid- and high-latitude land areas of the Northern Hemisphere. In the contiguous United States, the monthly mean temperature was near average in January.
Global Temperatures
The combined global land and ocean surface temperature was 1.53 degrees F (0.85 degrees C) warmer than the 20th century average of 53.6 degrees F (12.0 degrees C) for January based on preliminary data, surpassing the previous record set in 2002 at 1.28 degrees F (0.71 degrees C) above the average. Last month's record was greatly influenced by a record high land-surface temperature, which was 3.40 degrees F (1.89 degrees C) warmer than average. Separately, the global ocean-surface temperature was fourth warmest in the 128-year series, approximately 0.1 degrees F (0.05 degrees C) cooler than the record established during the very strong El Niño episode in 1998.
A moderate El Niño episode that began in September 2006 continued into January but weakened during the month. The presence of El Niño, along with the continuing global warming trend, contributed to the record warm January. Monthly mean temperatures more than 8 degrees F above average covered large parts of Eastern Europe and much of Russia, and temperatures more than 5 degrees F above average were widespread in Canada. The unusually warm conditions contributed to the 2nd lowest January snow cover extent on record for the Eurasian continent.
During the past century, global surface temperatures have increased at a rate near 0.11 degrees F (0.06 degrees C) per decade, but the rate of increase has been three times larger since 1976, or 0.32 degrees F (0.18 degrees C) per decade, with some of the largest temperature increases occurring in the high latitudes of the Northern Hemisphere.
Lucky us!
Here is graphic of the POAMA model.
CLICK ON THUMBNAIL
Graphic of models in pretty agreement of atleast Neutral threw the first half of the 2007 hurricane season.
See Graphic Here
what is SAL?
The Saharan Air Layer (SAL) is a mass of very dry, dusty air which forms over the Sahara Desert during the late spring, summer, and early fall and usually moves out over the tropical Atlantic Ocean. The SAL usually extends between 5,000-20,000 ft (1500-6000 m) in the atmosphere and is associated with large amounts of mineral dust, dry air (~50% less moisture than a typical tropical sounding), and strong winds (~25-55 mph or ~10-25 m/s).
The SAL has been shown to have significant negative impact on tropical cyclone intensity. Its dry air can act to weaken a tropical cyclone by inhibiting updrafts in the storm, while its strong winds can substantially increase the vertical wind shear in and around the storm environment. It is not yet clear what effect the SAL's dust has on tropical cyclone intensity, though some studies have suggested that it too may have a negative impact on intensification.
For futher questions you can always refer to Huricanes FAQ CIMSS-Saharan Air Layer Analysis
...COLD TONIGHT AND COOL WEEKEND ON TAP...
.DISCUSSION...SURFACE HIGH PRES DROPPING INTO THE GULF...COMBINED
WITH AN UPPER LEVEL DISTURBANCE ROUNDING INTO THE BASE OF THE UPPER
LEVEL TROUGH OVER NEW ENGLAND...WILL PUSH THE VEIL OF MID/HIGH
CLOUDS S OF THE S FLA AREA THIS EVENING. WITHERING BUT CONTINUING
DRY NW SURFACE WINDS WILL CONTINUE ADVECTING COOLER AND DRIER AIR
INTO S FLA. WINDS SHOULD WEAKEN LATER TONIGHT AND BECOME NEAR CLAM
IN THE INTERIOR AREAS. RADIATION COOLING WILL ALLOW FREEZING TEMPER
-ATURES MAINLY IN GLADES AND HENDRY COUNTIES...THOUGH N COLLIER IN
THE IMMOKALEE AREA AND IN W PALM BEACH WELL S AND SE OF LAKE OKEE
COULD SEE FREEZING TEMPS AS WELL. ELSEWHERE TEMPS WILL RANGE FROM
THE MID TO UPPER 30S INLAND TO THE LOWER TO MID 40S COAST. CONTINUED
COOL SAT AND SAT NIGHT THROUGH SOME CLOUDS AND MINIMAL POPS POSSIBLE
LATE SAT NIGHT INTO EARLY SUN AS ANOTHER FRONT MOVES THROUGH.
The temp today better not be a preview of summertime.
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