Historical CH4 mixing ratios from Law Dome (Antarctica) and Summit (Greenland) ice cores.
Source:  D.M. Etheridge
              L.P. Steele
              R.J. Francey
              R.L. Langenfelds
Historical CH4 record from DE08, DE08-2 and DSS ice cores (Antarctica).
Smoothed line through the ice core data (75 year smoothing spline).  From Figure 2.

   Mean Air Age                CH4 Mixing               Years Before
   (year A.D.)                 Ratio (ppb)               2000 A.D.
       1010                      667.1                       990
       1015                      668.7                       985
       1020                      670.3                       980
       1025                      671.8                       975
       1030                      673.1                       970
       1035                      674.4                       965
       1040                      675.4                       960
       1045                      676.3                       955
       1050                      676.9                       950
       1055                      677.3                       945
       1060                      677.5                       940
       1065                      677.5                       935
       1070                      677.5                       930
       1075                      677.4                       925
       1080                      677.3                       920
       1085                      677.2                       915
       1090                      677.2                       910
       1095                      677.4                       905
       1100                      677.7                       900
       1105                      678.2                       895
       1110                      678.9                       890
       1115                      679.7                       885
       1120                      680.6                       880
       1125                      681.6                       875
       1130                      682.5                       870
       1135                      683.5                       865
       1140                      684.4                       860
       1145                      685.2                       855
       1150                      685.9                       850
       1155                      686.5                       845
       1160                      686.9                       840
       1165                      687.0                       835
       1170                      687.0                       830
       1175                      686.8                       825
       1180                      686.4                       820
       1185                      685.6                       815
       1190                      684.7                       810
       1195                      683.4                       805
       1200                      681.8                       800
       1205                      679.9                       795
       1210                      677.8                       790
       1215                      675.6                       785
       1220                      673.3                       780
       1225                      671.0                       775
       1230                      668.7                       770
       1235                      666.6                       765
       1240                      664.7                       760
       1245                      663.1                       755
       1250                      661.9                       750
       1255                      661.1                       745
       1260                      660.6                       740
       1265                      660.4                       735
       1270                      660.5                       730
       1275                      660.9                       725
       1280                      661.5                       720
       1285                      662.3                       715
       1290                      663.2                       710
       1295                      664.3                       705
       1300                      665.5                       700
       1305                      666.7                       695
       1310                      667.9                       690
       1315                      669.1                       685
       1320                      670.3                       680 
       1325                      671.4                       675
       1330                      672.5                       670
       1335                      673.3                       665
       1340                      674.1                       660
       1345                      674.7                       655
       1350                      675.3                       650
       1355                      675.8                       645
       1360                      676.2                       640
       1365                      676.5                       635
       1370                      676.8                       630
       1375                      677.1                       625
       1380                      677.3                       620
       1385                      677.6                       615
       1390                      677.8                       610
       1395                      678.1                       605
       1400                      678.4                       600
       1405                      678.6                       595
       1410                      678.8                       590
       1415                      679.0                       585
       1420                      679.1                       580
       1425                      679.1                       575
       1430                      679.1                       570
       1435                      678.9                       565
       1440                      678.6                       560
       1445                      678.2                       555
       1450                      677.7                       550
       1455                      677.1                       545
       1460                      676.7                       540
       1465                      676.7                       535
       1470                      677.4                       530
       1475                      678.8                       525
       1480                      680.7                       520
       1485                      683.0                       515
       1490                      685.5                       510
       1495                      688.2                       505
       1500                      690.8                       500
       1505                      693.1                       495
       1510                      695.3                       490
       1515                      697.1                       485
       1520                      698.7                       480
       1525                      699.9                       475
       1530                      700.8                       470
       1535                      701.3                       465
       1540                      701.2                       460
       1545                      700.5                       455
       1550                      698.9                       450
       1555                      696.4                       445
       1560                      693.2                       440
       1565                      689.5                       435
       1570                      685.5                       430
       1575                      681.5                       425
       1580                      677.7                       420
       1585                      674.3                       415
       1590                      671.7                       410
       1595                      670.1                       405
       1600                      669.5                       400
       1605                      669.7                       395
       1610                      670.6                       390
       1615                      671.8                       385
       1620                      673.2                       380
       1625                      674.4                       375
       1630                      675.4                       370
       1635                      676.2                       365
       1640                      677.0                       360
       1645                      677.9                       355
       1650                      678.9                       350
       1655                      680.2                       345
       1660                      681.6                       340
       1665                      682.8                       335
       1670                      683.7                       330
       1675                      684.2                       325
       1680                      683.9                       320
       1685                      682.9                       315
       1690                      681.2                       310
       1695                      679.4                       305
       1700                      677.8                       300
       1705                      676.5                       295
       1710                      675.7                       290
       1715                      675.5                       285
       1720                      676.1                       280
       1725                      677.5                       275
       1730                      679.8                       270
       1735                      682.8                       265
       1740                      686.2                       260
       1745                      689.9                       255
       1750                      693.7                       250
       1755                      697.4                       245
       1760                      700.9                       240
       1765                      704.2                       235
       1770                      707.4                       230
       1775                      710.4                       225
       1780                      713.4                       220
       1785                      716.4                       215
       1790                      719.5                       210
       1795                      722.9                       205
       1800                      726.8                       200
       1805                      731.0                       195
       1810                      735.4                       190
       1815                      740.0                       185
       1820                      744.7                       180
       1825                      749.4                       175
       1830                      753.9                       170
       1835                      758.5                       165
       1840                      763.2                       160
       1845                      768.4                       155
       1850                      774.1                       150
       1855                      780.3                       145
       1860                      787.2                       140
       1865                      794.6                       135
       1870                      802.4                       130
       1875                      810.5                       125
       1880                      818.9                       120
       1885                      827.9                       115
       1890                      838.3                       110
       1895                      850.6                       105
       1900                      865.4                       100
       1905                      882.8                        95
       1910                      903.1                        90
       1915                      925.7                        85
       1920                      950.2                        80
       1925                      975.6                        75
       1930                     1001.0                        70
       1935                     1026.4                        65
       1940                     1052.6                        60
       1945                     1081.3                        55
       1950                     1115.7                        50
       1955                     1158.7                        45
       1960                     1211.1                        40
       1965                     1272.1                        35
       1970                     1339.8                        30
       1975                     1410.7                        25
       1980                     1482.2                        20                    
 

Global mixing ratios calculated over the last 1000 years from Antarctic and Greenland
measurements.
Smoothing spline fit with 75 year cut off (Table 2, part 1, Etheridge et al., 1998)
Also appears in IPCC WGI TAR Chapter 4 Figure 1 and Summary for Policymakers Figures 2
and 11, and as Figure 14-17 IN Chemistry of the Upper and Lower Atmosphere by Barbara J
Finlayson-Pitts and James N. Pitts Jr.  Academic Press, 2000.

  Mean Air Age               CH4 Mixing                  Years Before
   (year A.D.)               Ratio (ppb)                   2000 A.D.
       1010                     676.0                         990
       1020                     679.2                         980
       1030                     682.0                         970
       1040                     684.3                         960
       1050                     685.8                         950
       1060                     686.4                         940
       1070                     686.4                         930
       1080                     687.0                         920
       1090                     688.5                         910
       1100                     690.4                         900
       1110                     692.7                         890
       1120                     695.3                         880
       1130                     698.4                         870
       1140                     701.5                         860
       1150                     704.3                         850
       1160                     706.8                         840
       1170                     707.4                         830
       1180                     706.3                         820
       1190                     704.5                         810
       1200                     701.9                         800
       1210                     698.6                         790
       1220                     694.3                         780
       1230                     690.0                         770
       1240                     686.0                         760
       1250                     682.8                         750
       1260                     680.6                         740
       1270                     679.1                         730
       1280                     678.3                         720
       1290                     679.3                         710
       1300                     680.9                         700
       1310                     682.6                         690
       1320                     684.2                         680
       1330                     685.8                         670
       1340                     686.9                         660
       1350                     687.8                         650
       1360                     688.5                         640
       1370                     689.0                         630
       1380                     689.4                         620
       1390                     689.5                         610
       1400                     689.7                         600
       1410                     689.8                         590
       1420                     689.9                         580
       1430                     689.7                         570
       1440                     689.2                         560
       1450                     688.5                         550
       1460                     687.7                         540
       1470                     689.3                         530
       1480                     693.4                         520
       1490                     698.4                         510
       1500                     703.8                         500
       1510                     708.6                         490
       1520                     712.3                         480
       1530                     715.1                         470
       1540                     716.9                         460
       1550                     716.9                         450
       1560                     713.7                         440
       1570                     707.4                         430
       1580                     700.9                         420
       1590                     695.6                         410
       1600                     692.7                         400
       1610                     691.9                         390
       1620                     692.0                         380
       1630                     691.8                         370
       1640                     691.3                         360
       1650                     691.5                         350
       1660                     693.7                         340
       1670                     695.5                         330
       1680                     696.1                         320
       1690                     694.9                         310
       1700                     693.3                         300
       1710                     692.5                         290
       1720                     693.2                         280
       1730                     696.1                         270
       1740                     702.3                         260
       1750                     710.2                         250
       1760                     717.8                         240
       1770                     725.0                         230
       1780                     730.7                         220
       1790                     734.9                         210
       1800                     741.6                         200
       1810                     750.8                         190
       1820                     760.3                         180
       1830                     769.8                         170
       1840                     779.4                         160
       1850                     791.6                         150
       1860                     805.6                         140
       1870                     821.0                         130
       1880                     837.1                         120
       1890                     856.6                         110
Smoothing from here down has 12.5 year cut-off (Table 2 part 2 Etheridge et al., 1998)
       1900                     879.4                         100
       1902                     887.9                          98
       1904                     896.5                          96
       1906                     905.3                          94
       1908                     914.4                          92
       1910                     924.0                          90
       1912                     933.9                          88
       1914                     944.3                          86
       1916                     955.1                          84
       1918                     966.4                          82
       1920                     978.1                          80
       1922                     989.9                          78
       1924                    1001.8                          76
       1926                    1013.6                          74
       1928                    1025.0                          72
       1930                    1036.2                          70
       1932                    1047.2                          68
       1934                    1058.0                          66
       1936                    1068.7                          64
       1938                    1078.9                          62
       1940                    1088.9                          60
       1942                    1099.0                          58
       1944                    1109.4                          56
       1946                    1120.6                          54
       1948                    1133.2                          52
       1950                    1147.5                          50
       1952                    1163.8                          48
       1954                    1182.1                          46
       1956                    1202.4                          44
       1958                    1224.2                          42
       1960                    1247.5                          40
       1962                    1272.2                          38
       1964                    1298.4                          36
       1966                    1326.0                          34
       1968                    1355.3                          32
       1970                    1386.0                          30
       1972                    1417.5                          28
       1974                    1449.3                          26
       1976                    1481.5                          24
       1978                    1514.0                          22
       1980                    1547.1                          20
       1982                    1580.9                          18
       1984                    1614.2                          16
       1986                    1644.9                          14
       1988                    1671.6                          12
       1990                    1694.3                          10
       1992                    1714.0                           8