Jökull - 01.01.2011, Síða 12

P. Crochet and T. Jóhannesson
Table 5. 1961–1990 climatology: Statistics of differences between average temperature for the period 1961–
1990 calculated by Björnsson et al. (2007) and calculated from the daily fields derived with a constant lapse
rate. – Veðurfar 1961–1990: Töluleg dreifing á mismun á meðalhita tímabilsins 1961-1990. Annars vegar er
um að ræða hita sem reiknaður var af Halldóri Björnssyni o.fl. (2007) og hins vegar meðaltal daglega hitans
sem hér er lýst fyrir fast hitafall með hæð.
Statistics/Period Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean difference –0.35 –0.59 –0.79 –0.86 –0.56 –0.03 0.28 0.25 –0.07 –0.11 –0.26 –0.35
Mean absolute 0.52 0.71 0.87 0.95 0.71 0.39 0.52 0.45 0.30 0.35 0.45 0.52
difference
Root Mean Square 0.7 0.9 1.12 1.23 0.97 0.58 0.69 0.61 0.45 0.50 0.60 0.67
difference
Perc. difference 59.2 42.7 38.0 35.3 48.3 74.4 59.3 65.2 82.0 76.9 64.9 58.6
within ±0.5 ◦C
Perc. difference 86.8 72.7 64.1 60.3 72.6 92.0 87.9 90.3 95.3 94.1 91.3 87.2
within ±1 ◦C
Perc. difference 98.5 97.6 91.9 89.5 95.0 98.8 98.6 99.2 99.6 99.5 99.2 99.0
within ±2 ◦C
Table 6. 1961–1990 climatology: Statistics of differences between average temperature for the period 1961–
1990 calculated by Björnsson et al. (2007) and calculated from the daily fields derived with variable lapse rates.
– Veðurfar 1961–1990: Töluleg dreifing á mismun á meðalhita tímabilsins 1961-1990. Annars vegar er um að
ræða hita sem reiknaður var af Halldóri Björnssyni o.fl. (2007) og hins vegar meðaltal daglega hitans sem hér
er lýst fyrir breytilegt hitafall með hæð.
Statistics/Period Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean difference 0.81 0.53 0.08 0.30 0.45 0.46 0.49 0.43 0.29 0.43 0.79 0.69
Mean absolute difference 0.99 0.75 0.49 0.64 0.70 0.65 0.68 0.59 0.44 0.58 0.94 0.86
Root mean square difference 1.40 1.07 0.69 0.92 0.97 0.87 0.88 0.78 0.62 0.81 1.34 1.22
Percentage difference 42.5 51.3 65.4 55.6 50.0 49.9 45.2 52.6 67.2 57.2 44.1 46.7
within ±0.5 ◦C
Percentage difference 63.6 73.4 87.4 78.9 75.7 79.5 78.1 82.6 89.5 81.6 65.3 68.3
within ±1 ◦C
Percentage difference 85.4 91.5 98.0 94.8 94.2 96.3 96.6 97.9 99.0 96.9 86.5 88.4
within ±2 ◦C
period 1951–2010 (Figure 7). First, monthly fields
were derived by averaging daily temperature fields,
then annual fields were derived by averaging these
monthly fields and further averaged over the required
period. The 1950s had a positive anomaly ranging
mostly between 0.25 ◦C and 1 ◦C, then the anomaly
fluctuated between ±0.5 ◦C, either homogeneously or
with a dipole structure oriented N–S or NE–SW, until
1990 and becomes essentialy positive after that. The
last decade in particular has seen the largest warm-
ing over the last 60 years, above 1 ◦C with respect to
1961–1990 over a large part of the country. Long-
term climate warming in Iceland due to increasing
concentration of CO2 in the atmosphere is likely to
be 0.2 to 0.4 ◦C per decade until 2050 (Nawri and
Björnsson, 2010) but as can be seen from this analy-
sis, natural climate variability even on a decadal time-
scale can be larger than the effect of this estimated
long-term trend over several decades.
12 JÖKULL No. 61, 2011