| 1. |
- Arndt, D. S., et al.
(författare)
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State of the Climate in 2016
- 2017
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Ingår i: Bulletin of The American Meteorological Society - (BAMS). - 0003-0007 .- 1520-0477. ; 98:8, s. S1-S280
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Tidskriftsartikel (refereegranskat)abstract
- In 2016, the dominant greenhouse gases released into Earth's atmosphere-carbon dioxide, methane, and nitrous oxide-continued to increase and reach new record highs. The 3.5 +/- 0.1 ppm rise in global annual mean carbon dioxide from 2015 to 2016 was the largest annual increase observed in the 58-year measurement record. The annual global average carbon dioxide concentration at Earth's surface surpassed 400 ppm (402.9 +/- 0.1 ppm) for the first time in the modern atmospheric measurement record and in ice core records dating back as far as 800000 years. One of the strongest El Nino events since at least 1950 dissipated in spring, and a weak La Nina evolved later in the year. Owing at least in part to the combination of El Nino conditions early in the year and a long-term upward trend, Earth's surface observed record warmth for a third consecutive year, albeit by a much slimmer margin than by which that record was set in 2015. Above Earth's surface, the annual lower troposphere temperature was record high according to all datasets analyzed, while the lower stratospheric temperature was record low according to most of the in situ and satellite datasets. Several countries, including Mexico and India, reported record high annual temperatures while many others observed near-record highs. A week-long heat wave at the end of April over the northern and eastern Indian peninsula, with temperatures surpassing 44 degrees C, contributed to a water crisis for 330 million people and to 300 fatalities. In the Arctic the 2016 land surface temperature was 2.0 degrees C above the 1981-2010 average, breaking the previous record of 2007, 2011, and 2015 by 0.8 degrees C, representing a 3.5 degrees C increase since the record began in 1900. The increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 24 March, the sea ice extent at the end of the growth season saw its lowest maximum in the 37-year satellite record, tying with 2015 at 7.2% below the 1981-2010 average. The September 2016 Arctic sea ice minimum extent tied with 2007 for the second lowest value on record, 33% lower than the 1981-2010 average. Arctic sea ice cover remains relatively young and thin, making it vulnerable to continued extensive melt. The mass of the Greenland Ice Sheet, which has the capacity to contribute similar to 7 m to sea level rise, reached a record low value. The onset of its surface melt was the second earliest, after 2012, in the 37-year satellite record. Sea surface temperature was record high at the global scale, surpassing the previous record of 2015 by about 0.01 degrees C. The global sea surface temperature trend for the 21st century-to-date of +0.162 degrees C decade(-1) is much higher than the longer term 1950-2016 trend of +0.100 degrees C decade(-1). Global annual mean sea level also reached a new record high, marking the sixth consecutive year of increase. Global annual ocean heat content saw a slight drop compared to the record high in 2015. Alpine glacier retreat continued around the globe, and preliminary data indicate that 2016 is the 37th consecutive year of negative annual mass balance. Across the Northern Hemisphere, snow cover for each month from February to June was among its four least extensive in the 47-year satellite record. Continuing a pattern below the surface, record high temperatures at 20-m depth were measured at all permafrost observatories on the North Slope of Alaska and at the Canadian observatory on northernmost Ellesmere Island. In the Antarctic, record low monthly surface pressures were broken at many stations, with the southern annular mode setting record high index values in March and June. Monthly high surface pressure records for August and November were set at several stations. During this period, record low daily and monthly sea ice extents were observed, with the November mean sea ice extent more than 5 standard deviations below the 1981-2010 average. These record low sea ice values contrast sharply with the record high values observed during 2012-14. Over the region, springtime Antarctic stratospheric ozone depletion was less severe relative to the 1991-2006 average, but ozone levels were still low compared to pre-1990 levels. Closer to the equator, 93 named tropical storms were observed during 2016, above the 1981-2010 average of 82, but fewer than the 101 storms recorded in 2015. Three basins-the North Atlantic, and eastern and western North Pacific-experienced above-normal activity in 2016. The Australian basin recorded its least active season since the beginning of the satellite era in 1970. Overall, four tropical cyclones reached the Saffir-Simpson category 5 intensity level. The strong El Nino at the beginning of the year that transitioned to a weak La Nina contributed to enhanced precipitation variability around the world. Wet conditions were observed throughout the year across southern South America, causing repeated heavy flooding in Argentina, Paraguay, and Uruguay. Wetter-than-usual conditions were also observed for eastern Europe and central Asia, alleviating the drought conditions of 2014 and 2015 in southern Russia. In the United States, California had its first wetter-than-average year since 2012, after being plagued by drought for several years. Even so, the area covered by drought in 2016 at the global scale was among the largest in the post-1950 record. For each month, at least 12% of land surfaces experienced severe drought conditions or worse, the longest such stretch in the record. In northeastern Brazil, drought conditions were observed for the fifth consecutive year, making this the longest drought on record in the region. Dry conditions were also observed in western Bolivia and Peru; it was Bolivia's worst drought in the past 25 years. In May, with abnormally warm and dry conditions already prevailing over western Canada for about a year, the human-induced Fort McMurray wildfire burned nearly 590000 hectares and became the costliest disaster in Canadian history, with $3 billion (U.S. dollars) in insured losses.
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| 2. |
- Arndt, D. S., et al.
(författare)
-
STATE OF THE CLIMATE IN 2017
- 2018
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Ingår i: Bulletin of The American Meteorological Society - (BAMS). - : American Meteorological Society. - 0003-0007 .- 1520-0477. ; 99:8, s. S1-S310
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Forskningsöversikt (refereegranskat)
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| 3. |
- Aberg, Jan, et al.
(författare)
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Multialphabet coding with separate alphabet description
- 1997
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Ingår i: Proceedings. Compression and Complexity of SEQUENCES 1997. - 0818681322 ; , s. 56-65
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Konferensbidrag (refereegranskat)abstract
- For lossless universal source coding of memoryless sequences with an a priori unknown alphabet size (multialphabet coding), the alphabet of the sequence must be described as well as the sequence itself. Usually an efficient description of the alphabet can be made only by taking into account some additional information. We show that these descriptions can be separated in such a way that the encoding of the actual sequence can be performed independently of the alphabet description, and present sequential coding methods for such sequences. Such methods have applications in coding methods where the alphabet description is made available sequentially, such as PPM.
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| 4. |
- Aberg, J., et al.
(författare)
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Non-uniform PPM and context tree models
- 1998
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Ingår i: Proceedings DCC '98 Data Compression Conference. - 0818684062 ; , s. 279-288
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Konferensbidrag (refereegranskat)abstract
- The problem of optimizing PPM with the help of different choices of estimators and their parameters for different subsets of nodes in the context tree is considered. Methods of such optimization for Markov chain and context tree models for individual files and over given sets of files are presented, and it is demonstrated that the extension from Markov chain models to context tree models is necessary to receive significant improvements of the compression ratio.
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| 5. |
- Aberg, J., et al.
(författare)
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Towards understanding and improving escape probabilities in PPM
- 1997
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Ingår i: Proceedings DCC '97. Data Compression Conference. - 0818677619 ; , s. 22-31
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Konferensbidrag (refereegranskat)abstract
- The choice of expressions for the coding probabilities in general, and the escape probability in particular, is of great importance in the family of PPM algorithms. We present a parameterized version of the escape probability estimator which, together with a 'compactness' criterion, provides guidelines for the estimator design given a 'representative' set of files. This parameterization also makes it possible to adapt the expression of the escape probability during one-pass coding. Finally, we present results for one such compression scheme that illustrates the usefulness of our approach.
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| 6. |
- Smeets, B. J.M., et al.
(författare)
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Windmill generators a generalization and an observation of how many there are
- 1988
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Ingår i: Advances in Cryptology — EUROCRYPT 1988 - Workshop on the Theory and Application of Cryptographic Techniques, Proceedings. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1611-3349 .- 0302-9743. - 9783540502517 ; 330 LNCS, s. 325-330
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Konferensbidrag (refereegranskat)abstract
- The windmill technique has several practical advantageous over other techniques for high-speed generation or blockwise generation of pn-sequences. In this paper we generalize previous results by showing that if f(t)=α(t v) − β(t −v)t L is the minimal polynomial of a pn-sequence, then the sequence can be generated by a windmill generator. For L = 1,..127, and ν = 4, 8, 16 such that L ≡ ±3 mod 8 no irreducible polynomials f(t) were found. When L ≡ ±1 mod 8 the number of primitive f(t)’s was found to be approximately twice the expected number.
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| 7. |
- Smeets, B. J.M., et al.
(författare)
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Windmill pn-sequence generators
- 1989
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Ingår i: IEE Proceedings E: Computers and Digital Techniques. - : Institution of Engineering and Technology (IET). - 0143-7062. ; 136:5, s. 401-404
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Tidskriftsartikel (refereegranskat)abstract
- A windmill generator is a high-speed sequence generator capable of producing blocks of v consecutive symbols in parallel. It consists of v feedback-shift registers linked into a ring. The sequences are identical to those produced by a linear feedback-shift register with feedback polynomial of the special ('windmill') form f(t) = α(tv) - tLβ(t-v), where α(t) and β(t) are polynomials of degree less than L/v. L (relatively prime to v) is the degree of the polynomial, and is also the sum of the lengths of the registers making up the windmill. The connections of the windmill generator are specified by the coefficients of α(t) and β(t). The polynomial f(t) must be primitive if the output sequence is to be of maximal period. We have devised a search for windmill polynomials over the binary field that can generate sequences of period 2L - 1 in blocks of size v = 4,8, and 16, for L ranging over the odd values from 7 to 127. When L = ±3 mod 8, no irreductible windmill polynomials were found. For the other odd values of L, primitive windmill polynomials seem to occur about twice as frequently as would be expected from probabilistic considerations, so that they are in fact very common. For such values of L, roughly 2/L of all windmill polynomials with given v appear to be primitive.
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| 8. |
- Åberg, Jan, et al.
(författare)
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Estimation of escape probabilities for PPM based on universal source coding theory
- 1997
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Ingår i: Proceedings 1997 IEEE International Symposium on Information Theory. - 2157-8095. - 0780339568 - 9780780339569 ; , s. 65-65
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Konferensbidrag (refereegranskat)abstract
- Some of the best compression ratios for text compression are provided by the PPM (prediction by partial matching) class of algorithms. These algorithms are based on arithmetic coding using a fixed-depth Markov chain model of the source, i.e., the subsequence of symbols generated in any state s of the source is assumed to be the output of a memoryless subsource w=w(s). One of the most crucial steps in any PPM algorithm is the choice of the escape probability, i.e., the probability of a previously unseen symbol appearing in a given state. Let A={1,...,M} be the source alphabet, x/sup k/=x/ix/ i...x/sub k//spl isin/A/sup k/ be a sequence of symbols generated by a subsource w with unknown parameters, and m=m(x/sup k/) be the number of different symbols in x/sup k/. In most incarnations of PPM, the expression for the escape probability used for coding is of the form /spl thetav/(E|x/sup k/)=k(k,m)/k+m/spl alpha/+k(k,m), where /spl alpha/ is a parameter. The encoding of the "escape symbol" E is followed by a description of the new symbol, which is independent of the choice of escape probability and is not discussed. In almost all PPM schemes, the expression for the escape probability has been chosen heuristically. Following the method of Shtarkov et al. (see Probl. of Information Transmission, vol.31, no.2, p.20-35, 1995) we use a universal coding approach.
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